Add codecov badge to README.md

Author: l-x

README.md

@@ -3,6 +3,7 @@
 [![Monthly Downloads](https://poser.pugx.org/scn/hydrator/d/monthly)](https://packagist.org/packages/scn/hydrator)
 [![License](https://poser.pugx.org/scn/hydrator/license)](LICENSE)
 [![Build Status](https://travis-ci.org/SC-Networks/Hydrator.svg?branch=master)](https://travis-ci.org/SC-Networks/Hydrator)
+[![Coverage](https://codecov.io/gh/SC-Networks/Hydrator/branch/master/graph/badge.svg)](https://codecov.io/gh/SC-Networks/Hydrator)
 
 ## A pragmatic hydrator and extractor library