feat(README): Add coveralls badge to README

closes #728

Author: bmuenzenmeyer

README.md

@@ -1,6 +1,6 @@
 ![Pattern Lab Logo](/patternlab.png "Pattern Lab Logo")
 
-[![Build Status](https://travis-ci.org/pattern-lab/patternlab-node.svg?branch=master)](https://travis-ci.org/pattern-lab/patternlab-node) ![current release](https://img.shields.io/github/release/pattern-lab/patternlab-node.svg) ![license](https://img.shields.io/github/license/pattern-lab/patternlab-node.svg) [![Join the chat at Gitter](https://badges.gitter.im/pattern-lab/node.svg)](https://gitter.im/pattern-lab/node)
+[![Build Status](https://travis-ci.org/pattern-lab/patternlab-node.svg?branch=master)](https://travis-ci.org/pattern-lab/patternlab-node) ![current release](https://img.shields.io/github/release/pattern-lab/patternlab-node.svg) ![license](https://img.shields.io/github/license/pattern-lab/patternlab-node.svg) [![Coverage Status](https://coveralls.io/repos/github/pattern-lab/patternlab-node/badge.svg?branch=master)](https://coveralls.io/github/pattern-lab/patternlab-node?branch=master) [![Join the chat at Gitter](https://badges.gitter.im/pattern-lab/node.svg)](https://gitter.im/pattern-lab/node)
 
 # Pattern Lab Node Core