Add Code Climate badge

Created by: brynary

Hello,

I've just launched free-for-OSS support on Code Climate this week, and someone already added the gitlabhq project. Code Climate does hosted code quality metrics for Ruby apps. You can see the metrics here:

https://codeclimate.com/github/gitlabhq/gitlabhq

This is just a quick pull request that adds a link in the README, in case you want this to be readily available to contributors.

Let me know if you have any questions!

Best,

-Bryan