add Scrutinizer code coverage badges

Author: xabbuh

README.md

@@ -3,6 +3,7 @@ Xabbuh xApi Common
 
 [![Build Status](https://travis-ci.org/xabbuh/xapi-common.png?branch=master)](https://travis-ci.org/xabbuh/xapi-common)
 [![Scrutinizer Quality Score](https://scrutinizer-ci.com/g/xabbuh/xapi-common/badges/quality-score.png?s=b67412cc8bad9e86bd8fc24fdeb77a6bfea89a70)](https://scrutinizer-ci.com/g/xabbuh/xapi-common/)
+[![Code Coverage](https://scrutinizer-ci.com/g/xabbuh/xapi-common/badges/coverage.png?b=master)](https://scrutinizer-ci.com/g/xabbuh/xapi-common/?branch=master)
 
 Package providing common functionality for both server and client side implementations
 of the [Experience API](https://github.com/adlnet/xAPI-Spec/blob/master/xAPI.md).