Added document for overlayfs backend.
Added document for overlayfs backend.

Review: https://reviews.apache.org/r/44391/

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