Add badge for coverage status

README.md

@@ -2,6 +2,7 @@
 
 [![Software License](https://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat-square)](LICENSE.txt)
 [![Build Status](https://img.shields.io/travis/cakephp/cakephp/master.svg?style=flat-square)](https://travis-ci.org/cakephp/cakephp)
+[![Coverage Status](https://img.shields.io/coveralls/cakephp/cakephp/master.svg?style=flat-square)](https://coveralls.io/r/cakephp/cakephp?branch=master)
 [![Code Consistency](http://squizlabs.github.io/PHP_CodeSniffer/analysis/cakephp/cakephp/grade.svg)](http://squizlabs.github.io/PHP_CodeSniffer/analysis/cakephp/cakephp/)
 [![Total Downloads](https://img.shields.io/packagist/dt/cakephp/cakephp.svg?style=flat-square)](https://packagist.org/packages/cakephp/cakephp)