Add Code climate badge

Code Climate is a tool to analyse codebase quality in Ruby (and JavaScript) projects. I'm currently using it on some personal projects and it's been a great help so far when deciding what to refactor first.

This commit adds a badge linked to ai4r's Code Climate to the README. You can read some of the problems it found [in this list](https://codeclimate.com/github/SergioFierens/ai4r/code).