Documented the docker runtime isolator.
Documented the docker runtime isolator.

Review: https://reviews.apache.org/r/44189/

dforsyth/mesos,reneploetz/mesos,craimbert/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,verizonlabs/mesos,craimbert/mesos,chhsia0/mesos,abudnik/mesos,chhsia0/mesos,reneploetz/mesos,neilconway/mesos,asamerh4/mesos,craimbert/mesos,gsantovena/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,yuquanshan/customizedMesos,reneploetz/mesos,abudnik/mesos,neilconway/mesos,yuquanshan/customizedMesos,andschwa/mesos,chhsia0/mesos,reneploetz/mesos,reneploetz/mesos,zmalik/mesos,Gilbert88/mesos,jpeach/mesos,yuquanshan/customizedMesos,asamerh4/mesos,asamerh4/mesos,andschwa/mesos,andschwa/mesos,craimbert/mesos,jpeach/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,neilconway/mesos,kaysoky/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,zmalik/mesos,gsantovena/mesos,verizonlabs/mesos,neilconway/mesos,asamerh4/mesos,yuquanshan/customizedMesos,abudnik/mesos,jpeach/mesos,reneploetz/mesos,verizonlabs/mesos,gsantovena/mesos,andschwa/mesos,neilconway/mesos,Gilbert88/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,dforsyth/mesos,zmalik/mesos,kaysoky/mesos,Gilbert88/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,jpeach/mesos,zmalik/mesos,jpeach/mesos,kaysoky/mesos,asamerh4/mesos,asamerh4/mesos,neilconway/mesos,andschwa/mesos,zmalik/mesos,gsantovena/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,gsantovena/mesos,verizonlabs/mesos,abudnik/mesos,zmalik/mesos,dforsyth/mesos,kaysoky/mesos,dforsyth/mesos,dforsyth/mesos,craimbert/mesos,asamerh4/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,neilconway/mesos,Gilbert88/mesos,craimbert/mesos