Add Code Climate badge to README

Hey Norman,

Good seeing you in UY! Hope to run into you if you make it back to the US for RailsConf.

I thought you'd be interested to know that Haml was added to Code Climate a while back (in fact, you may have been the one who added it, hah):

https://codeclimate.com/github/haml/haml

This PR just adds a fancy, dynamic Code Climate badge to the README (like Travis), in case you'd like the quality reports to be available to project contributors.

Let me know what you think!

Best,

-Bryan