Add .gitreview file for gerrithub.io.

Install the git-review tool (`apt-get install git-review`) and then use
`git review -s` to set up your repo with branches, commit hooks, etc
for use with gerrit (you will probably need to enter your github
username).  Then upload patch sets with `git review`.  More info
at `git review --help`, naturally.

Change-Id: I86d57122098c45224125e7530fc99538726fe9b9