Update README.md

Author: troolee

README.md

@@ -2,6 +2,7 @@ gridstack.js
 ============
 
 [![Build Status](https://travis-ci.org/troolee/gridstack.js.svg?branch=master)](https://travis-ci.org/troolee/gridstack.js)
+[![Coverage Status](https://coveralls.io/repos/github/troolee/gridstack.js/badge.svg?branch=master)](https://coveralls.io/github/troolee/gridstack.js?branch=master)
 
 gridstack.js is a jQuery plugin for widget layout. This is drag-and-drop multi-column grid. It allows you to build
 draggable responsive bootstrap v3 friendly layouts. It also works great with [knockout.js](http://knockoutjs.com), [angular.js](https://angularjs.org) and touch devices.