Add Code Climate badge to README

Hey Everyone!

It looks like someone added Puma to Code Climate a few months back, and I thought you might want to know.

I work at Code Climate -- we do automated code reviews for Ruby and JavaScript using static analysis -- and we're completely free for open source. You can view your metrics here: https://codeclimate.com/github/puma/puma

In case you want this information to be available to contributors, this PR just adds a dynamic badge to the README (similar to Travis) that displays the code quality GPA

If you have any questions, let me know.

I hope you find Code Climate useful.

Cheers,
Mike