docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # Testing React components with Mocha test runner, jsdom, unexpected and SinonJS
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/akikoo/react-testing.svg)](https://greenkeeper.io/)
+
 This is just a quick basic setup of the above mentioned libraries. 
 
 [![Build Status](https://travis-ci.org/akikoo/react-testing.svg?branch=master)](https://travis-ci.org/akikoo/react-testing)