Add Code Climate badge to README

Hello,

It looks like someone (maybe you) added state_machine to Code Climate a while back, and I thought you might want to know.

Code Climate is a tool I built that does automated code reviews for Ruby using static analysis. It's free for OSS, and your metrics live here:

https://codeclimate.com/github/pluginaweek/state_machine

In case you want this information to be available to contributors, this PR just adds a dynamic badge to the README that displays the code quality GPA.

If you have any questions, let me know.

I hope you find Code Climate useful.

Cheers,
Bryan