Add Code Climate badge to README

Hi,

Just wanted to drop you a note to let you know that someone (perhaps one of you) added Better Errors to Code Climate a while back. (BTW, Better Errors is awesome! I wish it was built in to Rails.)

For those unfamiliar, Code Climate automatically reviews Ruby code for quality using static analysis. I built it and it's free for OSS. The metrics live here:

https://codeclimate.com/github/charliesome/better_errors

If you want this information to be available to contributors, this quick PR just adds a dynamic badge to the README (similar to Travis) that displays the code quality GPA.

I hope you like it. Let me know if I can help with anything.

Hope you find Code Climate useful.

Cheers,
Bryan