Add Code Climate badge to README

Hello there!

We at Code Climate noticed that Bourbon is currently collecting quality reports and is a 4.0! This pull request adds the badge to your README, a la Travis or other services.

The report for your repo is here:

https://codeclimate.com/github/thoughtbot/bourbon

If you don't like the badge placement, no worries, but continue to enjoy the service and we hope you keep up your awesome grade.

Thanks!

Mike