Add Code Climate badge to README

Hey,

Just wanted to drop you a note to let you know that someone added simple_form to Code Climate a while back. I'm the guy who built Code Climate, and it provides quality metrics for Ruby code, and made it completely free for OSS projects as a way to support them (like GitHub does).

The code quality reports live here:

https://codeclimate.com/github/plataformatec/simple_form

Also, we just launched fancy, dynamic badges for OSS projects.

In case you want this information to be available to contributors, this just adds a badge to the README (similar to Travis) that displays simple_form's quality GPA.

Let me know if you have any questions. Hope you find Code Climate useful.

Cheers,

-Bryan