Add coverage badge

Author: jwhitlock

README.rst

@@ -5,10 +5,14 @@ Pipeline
     :alt: Build Status
     :target: http://travis-ci.org/jazzband/django-pipeline
 
+.. image:: https://coveralls.io/repos/github/jazzband/django-pipeline/badge.svg?branch=master
+    :alt: Code Coverage
+    :target: https://coveralls.io/github/jazzband/django-pipeline?branch=master
+
 .. image:: https://jazzband.co/static/img/badge.svg
     :alt: Jazzband
     :target: https://jazzband.co/
-   
+
 .. image:: https://badge.fury.io/py/django-pipeline.svg
     :alt: PYPI
     :target: https://badge.fury.io/py/django-pipeline