Add code climate coverage badge to README

Author: krzysiek1507

README.md

@@ -2,6 +2,7 @@
 [![Build Status](https://img.shields.io/travis/reactjs/react-rails/master.svg?style=flat-square)](https://travis-ci.org/reactjs/react-rails)
 [![Gemnasium](https://img.shields.io/gemnasium/reactjs/react-rails.svg?style=flat-square)](https://gemnasium.com/reactjs/react-rails)
 [![Code Climate](https://img.shields.io/codeclimate/github/reactjs/react-rails.svg?style=flat-square)](https://codeclimate.com/github/reactjs/react-rails)
+[![Test Coverage](https://img.shields.io/codeclimate/coverage/github/reactjs/react-rails.svg?style=flat-square)](https://codeclimate.com/github/reactjs/react-rails/coverage)
 
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