Update HACKING with update git aliases.
Merge pull request #151 from mitechie/update-git-docs

- Update the fetch-pr alias per suggestion from Gary to add the +ref.
- Rename the juju-sync to sync-juju to start the trend of available sync-* commands
- Add a sync-trunk which will do a rebase from your develop branch.
- Update docs on how to keep your feature branch up to date based on the latest upstream trunk.