Add codeclimate badge

Hi,

I've been trying to create a similar library like this recently, but `mutations` seems already has everything I've had in mind so far, I mean literally, how could this be possible?

To show my appreciation here is this little PR to add a codeclimate badge, feel free to take it or leave it, it's totally up to you.

I will try to use `mutations` in my new projects, hopefully along the way I might be able to report some issues or send some actually useful pull requests.

Thank you very much for sharing this gem :+1: