Update README
Update README

- Remove upgrading from 2015.2 section
- Remove testing zack/r10k section
- Reword adding webhook section
- Misc improvements

npwalker/control-repo,AzureCAT-GSI/control-repo,drewablank/symlink,fvoges/rampup-control-repo-test,AzureCAT-GSI/control-repo,fvoges/rampup-control-repo-test,drewablank/symlink,npwalker/control-repo