Updated the markdown style guide with headings capitalization.
Updated the markdown style guide with headings capitalization.

Review: https://reviews.apache.org/r/53614/

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