Improve the bash completion script installation instructions.
Improve the bash completion script installation instructions.

Summary:
On some distributions there is a specific folder in which the script should
be placed, rather than adding the `source` command in the user profile.

https://github.com/facebook/buck/pull/56

Test Plan: Follow the instructions and make sure it works as expected.

clonetwin26/buck,marcinkwiatkowski/buck,artiya4u/buck,bocon13/buck,hgl888/buck,grumpyjames/buck,romanoid/buck,vschs007/buck,SeleniumHQ/buck,clonetwin26/buck,kageiit/buck,lukw00/buck,rmaz/buck,brettwooldridge/buck,JoelMarcey/buck,Heart2009/buck,thinkernel/buck,brettwooldridge/buck,rmaz/buck,SeleniumHQ/buck,stuhood/buck,daedric/buck,neonichu/buck,Dominator008/buck,LegNeato/buck,Learn-Android-app/buck,vschs007/buck,vine/buck,zhuxiaohao/buck,Learn-Android-app/buck,shs96c/buck,shs96c/buck,dpursehouse/buck,ilya-klyuchnikov/buck,rmaz/buck,tgummerer/buck,darkforestzero/buck,JoelMarcey/buck,zhan-xiong/buck,MarkRunWu/buck,dushmis/buck,rmaz/buck,jackminicloud/buck,jackminicloud/buck,illicitonion/buck,zhan-xiong/buck,k21/buck,ilya-klyuchnikov/buck,tgummerer/buck,1yvT0s/buck,justinmuller/buck,darkforestzero/buck,davido/buck,Dominator008/buck,nguyentruongtho/buck,mread/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,LegNeato/buck,bocon13/buck,shs96c/buck,OkBuilds/buck,k21/buck,zpao/buck,GerritCodeReview/buck,sdwilsh/buck,tgummerer/buck,shybovycha/buck,darkforestzero/buck,romanoid/buck,luiseduardohdbackup/buck,belomx/open_tools,Learn-Android-app/buck,illicitonion/buck,romanoid/buck,facebook/buck,vine/buck,raviagarwal7/buck,rhencke/buck,zhan-xiong/buck,facebook/buck,1yvT0s/buck,mread/buck,grumpyjames/buck,siddhartharay007/buck,romanoid/buck,JoelMarcey/buck,daedric/buck,illicitonion/buck,dushmis/buck,siddhartharay007/buck,zhan-xiong/buck,MarkRunWu/buck,dushmis/buck,robbertvanginkel/buck,jackminicloud/buck,JoelMarcey/buck,JoelMarcey/buck,stuhood/buck,kageiit/buck,nguyentruongtho/buck,darkforestzero/buck,zpao/buck,neonichu/buck,janicduplessis/buck,illicitonion/buck,darkforestzero/buck,shs96c/buck,siddhartharay007/buck,rmaz/buck,ilya-klyuchnikov/buck,artiya4u/buck,Heart2009/buck,sdwilsh/buck,janicduplessis/buck,nguyentruongtho/buck,brettwooldridge/buck,shs96c/buck,robbertvanginkel/buck,vschs007/buck,JoelMarcey/buck,pwz3n0/buck,mnuessler/buck,stuhood/buck,sdwilsh/buck,mikekap/buck,vschs007/buck,lukw00/buck,davido/buck,Addepar/buck,dsyang/buck,clonetwin26/buck,hgl888/buck,hgl888/buck,rowillia/buck,justinmuller/buck,dsyang/buck,siddhartharay007/buck,SeleniumHQ/buck,zpao/buck,bocon13/buck,brettwooldridge/buck,daedric/buck,artiya4u/buck,raviagarwal7/buck,vschs007/buck,marcinkwiatkowski/buck,neonichu/buck,rowillia/buck,SeleniumHQ/buck,zhuxiaohao/buck,tgummerer/buck,vschs007/buck,k21/buck,rhencke/buck,Addepar/buck,marcinkwiatkowski/buck,janicduplessis/buck,k21/buck,mogers/buck,1yvT0s/buck,mogers/buck,grumpyjames/buck,facebook/buck,zhan-xiong/buck,artiya4u/buck,vine/buck,hgl888/buck,dushmis/buck,LegNeato/buck,lukw00/buck,hgl888/buck,dpursehouse/buck,LegNeato/buck,hgl888/buck,artiya4u/buck,k21/buck,JoelMarcey/buck,shybovycha/buck,raviagarwal7/buck,mikekap/buck,robbertvanginkel/buck,illicitonion/buck,artiya4u/buck,shybovycha/buck,davido/buck,mikekap/buck,liuyang-li/buck,vschs007/buck,lukw00/buck,raviagarwal7/buck,illicitonion/buck,darkforestzero/buck,rowillia/buck,hgl888/buck,artiya4u/buck,darkforestzero/buck,Addepar/buck,mogers/buck,daedric/buck,mikekap/buck,marcinkwiatkowski/buck,janicduplessis/buck,facebook/buck,shs96c/buck,Heart2009/buck,daedric/buck,OkBuilds/buck,dushmis/buck,robbertvanginkel/buck,tgummerer/buck,lukw00/buck,JoelMarcey/buck,luiseduardohdbackup/buck,vschs007/buck,kageiit/buck,rmaz/buck,daedric/buck,dsyang/buck,pwz3n0/buck,janicduplessis/buck,Dominator008/buck,janicduplessis/buck,saleeh93/buck-cutom,OkBuilds/buck,OkBuilds/buck,dsyang/buck,justinmuller/buck,davido/buck,stuhood/buck,Heart2009/buck,darkforestzero/buck,rowillia/buck,1yvT0s/buck,zhuxiaohao/buck,dushmis/buck,rhencke/buck,janicduplessis/buck,SeleniumHQ/buck,jackminicloud/buck,zhuxiaohao/buck,mikekap/buck,rhencke/buck,brettwooldridge/buck,stuhood/buck,nguyentruongtho/buck,vine/buck,kageiit/buck,mikekap/buck,Addepar/buck,k21/buck,Learn-Android-app/buck,thinkernel/buck,pwz3n0/buck,grumpyjames/buck,clonetwin26/buck,marcinkwiatkowski/buck,Dominator008/buck,Distrotech/buck,stuhood/buck,Heart2009/buck,Distrotech/buck,LegNeato/buck,dpursehouse/buck,grumpyjames/buck,pwz3n0/buck,ilya-klyuchnikov/buck,bocon13/buck,dsyang/buck,vschs007/buck,davido/buck,GerritCodeReview/buck,zhan-xiong/buck,shybovycha/buck,1yvT0s/buck,zhuxiaohao/buck,vschs007/buck,justinmuller/buck,nguyentruongtho/buck,zhan-xiong/buck,1yvT0s/buck,belomx/open_tools,OkBuilds/buck,dsyang/buck,dpursehouse/buck,siddhartharay007/buck,Distrotech/buck,marcinkwiatkowski/buck,rowillia/buck,SeleniumHQ/buck,JoelMarcey/buck,Addepar/buck,liuyang-li/buck,shs96c/buck,grumpyjames/buck,justinmuller/buck,shybovycha/buck,artiya4u/buck,illicitonion/buck,davido/buck,daedric/buck,marcinkwiatkowski/buck,stuhood/buck,Addepar/buck,shybovycha/buck,clonetwin26/buck,saleeh93/buck-cutom,rhencke/buck,vschs007/buck,dpursehouse/buck,pwz3n0/buck,romanoid/buck,mikekap/buck,clonetwin26/buck,clonetwin26/buck,zpao/buck,shybovycha/buck,illicitonion/buck,bocon13/buck,Distrotech/buck,mnuessler/buck,siddhartharay007/buck,brettwooldridge/buck,liuyang-li/buck,zpao/buck,shs96c/buck,Dominator008/buck,Heart2009/buck,kageiit/buck,romanoid/buck,mogers/buck,hgl888/buck,LegNeato/buck,rhencke/buck,mnuessler/buck,JoelMarcey/buck,1yvT0s/buck,mread/buck,marcinkwiatkowski/buck,ilya-klyuchnikov/buck,dsyang/buck,robbertvanginkel/buck,rhencke/buck,justinmuller/buck,tgummerer/buck,romanoid/buck,facebook/buck,Addepar/buck,illicitonion/buck,Heart2009/buck,romanoid/buck,brettwooldridge/buck,mread/buck,rmaz/buck,dsyang/buck,dsyang/buck,LegNeato/buck,liuyang-li/buck,dsyang/buck,dpursehouse/buck,liuyang-li/buck,OkBuilds/buck,raviagarwal7/buck,mikekap/buck,daedric/buck,Dominator008/buck,bocon13/buck,OkBuilds/buck,davido/buck,Distrotech/buck,artiya4u/buck,artiya4u/buck,Dominator008/buck,luiseduardohdbackup/buck,mnuessler/buck,brettwooldridge/buck,Dominator008/buck,Distrotech/buck,shybovycha/buck,ilya-klyuchnikov/buck,darkforestzero/buck,hgl888/buck,Heart2009/buck,kageiit/buck,mnuessler/buck,neonichu/buck,shybovycha/buck,ilya-klyuchnikov/buck,rhencke/buck,Addepar/buck,JoelMarcey/buck,raviagarwal7/buck,mogers/buck,Distrotech/buck,zpao/buck,raviagarwal7/buck,ilya-klyuchnikov/buck,luiseduardohdbackup/buck,raviagarwal7/buck,siddhartharay007/buck,mikekap/buck,kageiit/buck,davido/buck,stuhood/buck,saleeh93/buck-cutom,ilya-klyuchnikov/buck,dushmis/buck,tgummerer/buck,janicduplessis/buck,clonetwin26/buck,k21/buck,daedric/buck,lukw00/buck,Learn-Android-app/buck,facebook/buck,rhencke/buck,dushmis/buck,sdwilsh/buck,raviagarwal7/buck,belomx/open_tools,JoelMarcey/buck,dpursehouse/buck,marcinkwiatkowski/buck,dpursehouse/buck,nguyentruongtho/buck,romanoid/buck,dushmis/buck,neonichu/buck,robbertvanginkel/buck,janicduplessis/buck,justinmuller/buck,daedric/buck,vine/buck,mogers/buck,thinkernel/buck,rowillia/buck,k21/buck,grumpyjames/buck,Distrotech/buck,OkBuilds/buck,robbertvanginkel/buck,luiseduardohdbackup/buck,OkBuilds/buck,mread/buck,zpao/buck,vine/buck,pwz3n0/buck,MarkRunWu/buck,bocon13/buck,illicitonion/buck,k21/buck,k21/buck,SeleniumHQ/buck,dushmis/buck,mogers/buck,Learn-Android-app/buck,1yvT0s/buck,belomx/open_tools,darkforestzero/buck,SeleniumHQ/buck,grumpyjames/buck,liuyang-li/buck,tgummerer/buck,mogers/buck,tgummerer/buck,bocon13/buck,pwz3n0/buck,zhan-xiong/buck,mogers/buck,dsyang/buck,mnuessler/buck,JoelMarcey/buck,liuyang-li/buck,Dominator008/buck,GerritCodeReview/buck,justinmuller/buck,Distrotech/buck,LegNeato/buck,sdwilsh/buck,MarkRunWu/buck,darkforestzero/buck,neonichu/buck,ilya-klyuchnikov/buck,sdwilsh/buck,davido/buck,vine/buck,k21/buck,lukw00/buck,LegNeato/buck,shs96c/buck,pwz3n0/buck,Addepar/buck,rowillia/buck,vschs007/buck,zhan-xiong/buck,sdwilsh/buck,justinmuller/buck,dsyang/buck,zhuxiaohao/buck,Learn-Android-app/buck,saleeh93/buck-cutom,mnuessler/buck,rowillia/buck,artiya4u/buck,rhencke/buck,stuhood/buck,robbertvanginkel/buck,mikekap/buck,tgummerer/buck,OkBuilds/buck,sdwilsh/buck,Addepar/buck,MarkRunWu/buck,grumpyjames/buck,sdwilsh/buck,lukw00/buck,SeleniumHQ/buck,sdwilsh/buck,brettwooldridge/buck,davido/buck,zhan-xiong/buck,marcinkwiatkowski/buck,luiseduardohdbackup/buck,bocon13/buck,clonetwin26/buck,mnuessler/buck,vschs007/buck,Learn-Android-app/buck,LegNeato/buck,grumpyjames/buck,daedric/buck,neonichu/buck,marcinkwiatkowski/buck,shs96c/buck,pwz3n0/buck,marcinkwiatkowski/buck,robbertvanginkel/buck,Dominator008/buck,bocon13/buck,vine/buck,LegNeato/buck,saleeh93/buck-cutom,luiseduardohdbackup/buck,davido/buck,MarkRunWu/buck,rmaz/buck,brettwooldridge/buck,rmaz/buck,liuyang-li/buck,rmaz/buck,robbertvanginkel/buck,LegNeato/buck,rowillia/buck,thinkernel/buck,daedric/buck,dsyang/buck,Distrotech/buck,stuhood/buck,romanoid/buck,clonetwin26/buck,raviagarwal7/buck,rmaz/buck,illicitonion/buck,liuyang-li/buck,pwz3n0/buck,zhan-xiong/buck,bocon13/buck,tgummerer/buck,clonetwin26/buck,SeleniumHQ/buck,luiseduardohdbackup/buck,brettwooldridge/buck,Dominator008/buck,k21/buck,SeleniumHQ/buck,rhencke/buck,Addepar/buck,justinmuller/buck,k21/buck,siddhartharay007/buck,shs96c/buck,mikekap/buck,shybovycha/buck,liuyang-li/buck,bocon13/buck,justinmuller/buck,neonichu/buck,sdwilsh/buck,neonichu/buck,vine/buck,OkBuilds/buck,rowillia/buck,Addepar/buck,zhan-xiong/buck,zhuxiaohao/buck,luiseduardohdbackup/buck,facebook/buck,lukw00/buck,davido/buck,illicitonion/buck,ilya-klyuchnikov/buck,Addepar/buck,shs96c/buck,MarkRunWu/buck,janicduplessis/buck,ilya-klyuchnikov/buck,Learn-Android-app/buck,illicitonion/buck,Learn-Android-app/buck,LegNeato/buck,siddhartharay007/buck,Distrotech/buck,clonetwin26/buck,mogers/buck,raviagarwal7/buck,Dominator008/buck,zhuxiaohao/buck,brettwooldridge/buck,grumpyjames/buck,vine/buck,vine/buck,rmaz/buck,mnuessler/buck,ilya-klyuchnikov/buck,stuhood/buck,justinmuller/buck,romanoid/buck,shybovycha/buck,1yvT0s/buck,rowillia/buck,romanoid/buck,darkforestzero/buck,grumpyjames/buck,raviagarwal7/buck,janicduplessis/buck,Heart2009/buck,saleeh93/buck-cutom,raviagarwal7/buck,rowillia/buck,robbertvanginkel/buck,zhuxiaohao/buck,mikekap/buck,romanoid/buck,justinmuller/buck,GerritCodeReview/buck,SeleniumHQ/buck,MarkRunWu/buck,robbertvanginkel/buck,janicduplessis/buck,lukw00/buck,sdwilsh/buck,OkBuilds/buck,nguyentruongtho/buck,sdwilsh/buck,rmaz/buck,shs96c/buck,darkforestzero/buck,clonetwin26/buck,shybovycha/buck,brettwooldridge/buck,pwz3n0/buck,Learn-Android-app/buck,OkBuilds/buck,mread/buck,lukw00/buck,liuyang-li/buck,shybovycha/buck,SeleniumHQ/buck,davido/buck,zhan-xiong/buck,daedric/buck,mogers/buck