docs: coverage badges

added coverage badges to README

Author: dkhurshudian

README.MD

@@ -3,6 +3,7 @@
 master|alpha
 ------|-----
 [![Build Status](https://travis-ci.org/alephdata/vis2.js.svg?branch=master)](https://travis-ci.org/alephdata/vis2.js)|[![Build Status](https://travis-ci.org/alephdata/vis2.js.svg?branch=alpha)](https://travis-ci.org/alephdata/vis2.js)
+[![Coverage Status](https://coveralls.io/repos/github/alephdata/vis2.js/badge.svg?branch=master)](https://coveralls.io/github/alephdata/vis2.js?branch=master) | [![Coverage Status](https://coveralls.io/repos/github/alephdata/vis2.js/badge.svg?branch=master)](https://coveralls.io/github/alephdata/vis2.js?branch=alpha)
 
 
 ### Prerequisites