Add instructions on sharing macros to docs
Add instructions on sharing macros to docs

Summary:
I was recently trying to get some macros setup for a project and I couldn't remember how includes were supposed to work, and it would have been helpful if the page on macros was more instructive. This is just adding a section to introduce the concept of project includes so it's easier to come across/is in more than one place.

Also add some section headers to break up the article.
Closes https://github.com/facebook/buck/pull/1065

Reviewed By: ttsugriy

Pulled By: ttsugriy

fbshipit-source-id: f54d673

robbertvanginkel/buck,robbertvanginkel/buck,rmaz/buck,shybovycha/buck,shs96c/buck,zhan-xiong/buck,robbertvanginkel/buck,kageiit/buck,zhan-xiong/buck,robbertvanginkel/buck,brettwooldridge/buck,shybovycha/buck,LegNeato/buck,LegNeato/buck,dsyang/buck,dsyang/buck,dsyang/buck,romanoid/buck,romanoid/buck,zpao/buck,facebook/buck,clonetwin26/buck,shs96c/buck,shybovycha/buck,k21/buck,SeleniumHQ/buck,nguyentruongtho/buck,Addepar/buck,rmaz/buck,zhan-xiong/buck,rmaz/buck,shs96c/buck,shybovycha/buck,LegNeato/buck,shs96c/buck,JoelMarcey/buck,SeleniumHQ/buck,SeleniumHQ/buck,romanoid/buck,zhan-xiong/buck,zpao/buck,brettwooldridge/buck,shybovycha/buck,SeleniumHQ/buck,zhan-xiong/buck,facebook/buck,facebook/buck,romanoid/buck,Addepar/buck,JoelMarcey/buck,LegNeato/buck,shybovycha/buck,clonetwin26/buck,LegNeato/buck,ilya-klyuchnikov/buck,SeleniumHQ/buck,Addepar/buck,rmaz/buck,shybovycha/buck,brettwooldridge/buck,Addepar/buck,SeleniumHQ/buck,ilya-klyuchnikov/buck,k21/buck,kageiit/buck,brettwooldridge/buck,clonetwin26/buck,shybovycha/buck,robbertvanginkel/buck,romanoid/buck,shs96c/buck,ilya-klyuchnikov/buck,kageiit/buck,dsyang/buck,zhan-xiong/buck,ilya-klyuchnikov/buck,shybovycha/buck,k21/buck,zhan-xiong/buck,zpao/buck,nguyentruongtho/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,zpao/buck,clonetwin26/buck,k21/buck,clonetwin26/buck,nguyentruongtho/buck,nguyentruongtho/buck,shs96c/buck,rmaz/buck,romanoid/buck,ilya-klyuchnikov/buck,ilya-klyuchnikov/buck,dsyang/buck,clonetwin26/buck,JoelMarcey/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,nguyentruongtho/buck,dsyang/buck,shybovycha/buck,ilya-klyuchnikov/buck,k21/buck,kageiit/buck,Addepar/buck,LegNeato/buck,LegNeato/buck,brettwooldridge/buck,shs96c/buck,SeleniumHQ/buck,facebook/buck,brettwooldridge/buck,ilya-klyuchnikov/buck,robbertvanginkel/buck,k21/buck,LegNeato/buck,SeleniumHQ/buck,JoelMarcey/buck,ilya-klyuchnikov/buck,facebook/buck,romanoid/buck,kageiit/buck,clonetwin26/buck,dsyang/buck,brettwooldridge/buck,facebook/buck,Addepar/buck,shybovycha/buck,k21/buck,dsyang/buck,ilya-klyuchnikov/buck,shybovycha/buck,robbertvanginkel/buck,SeleniumHQ/buck,zhan-xiong/buck,Addepar/buck,rmaz/buck,brettwooldridge/buck,rmaz/buck,SeleniumHQ/buck,shybovycha/buck,shs96c/buck,JoelMarcey/buck,robbertvanginkel/buck,romanoid/buck,brettwooldridge/buck,shs96c/buck,romanoid/buck,k21/buck,romanoid/buck,zhan-xiong/buck,Addepar/buck,rmaz/buck,dsyang/buck,rmaz/buck,k21/buck,rmaz/buck,nguyentruongtho/buck,dsyang/buck,robbertvanginkel/buck,k21/buck,Addepar/buck,Addepar/buck,shs96c/buck,dsyang/buck,JoelMarcey/buck,JoelMarcey/buck,LegNeato/buck,romanoid/buck,JoelMarcey/buck,robbertvanginkel/buck,LegNeato/buck,Addepar/buck,ilya-klyuchnikov/buck,shs96c/buck,clonetwin26/buck,shs96c/buck,LegNeato/buck,clonetwin26/buck,SeleniumHQ/buck,zhan-xiong/buck,dsyang/buck,k21/buck,SeleniumHQ/buck,LegNeato/buck,brettwooldridge/buck,nguyentruongtho/buck,JoelMarcey/buck,robbertvanginkel/buck,Addepar/buck,clonetwin26/buck,romanoid/buck,JoelMarcey/buck,brettwooldridge/buck,clonetwin26/buck,robbertvanginkel/buck,dsyang/buck,JoelMarcey/buck,kageiit/buck,shs96c/buck,LegNeato/buck,zhan-xiong/buck,zhan-xiong/buck,k21/buck,kageiit/buck,rmaz/buck,brettwooldridge/buck,zpao/buck,Addepar/buck,rmaz/buck,zpao/buck,zhan-xiong/buck,romanoid/buck,JoelMarcey/buck,k21/buck,JoelMarcey/buck,zpao/buck,clonetwin26/buck,rmaz/buck,facebook/buck,clonetwin26/buck,SeleniumHQ/buck