Added overview section to `docs/upgrades.md`.
Added overview section to `docs/upgrades.md`.

Review: https://reviews.apache.org/r/43798/

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