Display code coverage on README.

Author: BernardoSilva

README.md

@@ -1,6 +1,6 @@
 # PHP Enum implementation inspired from SplEnum
 
-[![Build Status](https://travis-ci.org/myclabs/php-enum.png?branch=master)](https://travis-ci.org/myclabs/php-enum) [![Latest Stable Version](https://poser.pugx.org/myclabs/php-enum/version.png)](https://packagist.org/packages/myclabs/php-enum) [![Total Downloads](https://poser.pugx.org/myclabs/php-enum/downloads.png)](https://packagist.org/packages/myclabs/php-enum)
+[![Build Status](https://travis-ci.org/myclabs/php-enum.png?branch=master)](https://travis-ci.org/myclabs/php-enum) [![Latest Stable Version](https://poser.pugx.org/myclabs/php-enum/version.png)](https://packagist.org/packages/myclabs/php-enum) [![Total Downloads](https://poser.pugx.org/myclabs/php-enum/downloads.png)](https://packagist.org/packages/myclabs/php-enum) [![Code Coverage](https://scrutinizer-ci.com/g/myclabs/php-enum/badges/coverage.png?b=master)](https://scrutinizer-ci.com/g/myclabs/php-enum/?branch=master)
 
 ## Why?