Add coverage badge to our README

README.md

@@ -3,7 +3,7 @@
 
 [![Build Status](https://travis-ci.org/ebassi/graphene.svg?branch=master)](https://travis-ci.org/ebassi/graphene)
 [![Build status](https://ci.appveyor.com/api/projects/status/pw7o5grgko1l06hd/branch/master?svg=true)](https://ci.appveyor.com/project/ebassi/graphene/branch/master)
-
+[![Coverage Status](https://coveralls.io/repos/github/ebassi/graphene/badge.svg?branch=master)](https://coveralls.io/github/ebassi/graphene?branch=master)
 
 When creating graphic libraries you most likely end up dealing with points
 and rectangles. If you're particularly unlucky, you may end up dealing