Add code coverage to README.

README.md

@@ -1,6 +1,7 @@
 ## Django Archive
 
 [![Build Status](https://travis-ci.org/nathan-osman/django-archive.svg?branch=master)](https://travis-ci.org/nathan-osman/django-archive)
+[![Coverage Status](https://coveralls.io/repos/github/nathan-osman/django-archive/badge.svg?branch=master)](https://coveralls.io/github/nathan-osman/django-archive?branch=master)
 [![Latest Documentation](https://readthedocs.org/projects/django-archive/badge/?version=latest)](http://django-archive.readthedocs.org/en/latest/)
 [![PyPI Version](http://img.shields.io/pypi/v/django-archive.svg?style=flat)](https://pypi.python.org/pypi/django-archive)
 [![PyPI Downloads](http://img.shields.io/pypi/dm/django-archive.svg?style=flat)](https://pypi.python.org/pypi/django-archive)