Add Code Climate badge to README

Hey dB.,

Hope things are going well. I thought you might be interested to know that someone added Grape to Code Climate (perhaps you ;-) ): 

https://codeclimate.com/github/intridea/grape

I saw you link to Travis CI from the README, and we recently launched fancy new badges, so I thought I'd whip up a quick PR in case you'd like the quality reports to be available to project contributors.

Any/all feedback appreciated, of course. Hope to see you around in NYC sometime soon.

Best,

-Bryan