Add Coveralls badge

README.md

@@ -1,5 +1,6 @@
 [![Build Status](https://travis-ci.org/nanoc/nanoc.png)](https://travis-ci.org/nanoc/nanoc)
 [![Code Climate](https://codeclimate.com/github/nanoc/nanoc.png)](https://codeclimate.com/github/nanoc/nanoc)
+[![Coverage Status](https://coveralls.io/repos/nanoc/nanoc/badge.png?branch=master)](https://coveralls.io/r/nanoc/nanoc)
 
 **Please take a moment and [donate](http://pledgie.com/campaigns/9282) to nanoc. A lot of time has gone into developing nanoc, and I would like to keep the current pace. Your support will ensure that nanoc will continue to improve.**