Adding badge to README

Author: nicoddemus

README.rst

@@ -15,20 +15,21 @@ of a test:
         UnixFS.rm('file')
         os.remove.assert_called_once_with('file')
         
-|version| |downloads| |ci|
+|version| |downloads| |ci| |coverage|
 
 .. |version| image:: http://img.shields.io/pypi/v/pytest-mock.png
   :target: https://crate.io/packages/pytest-mock
 
 .. |downloads| image:: http://img.shields.io/pypi/dm/pytest-mock.png
   :target: https://crate.io/packages/pytest-mock
-  
+
 .. |ci| image:: http://img.shields.io/travis/nicoddemus/pytest-mock.png
   :target: https://travis-ci.org/nicoddemus/pytest-mock
 
 .. |coverage| image:: http://img.shields.io/coveralls/nicoddemus/pytest-mock.png
   :target: https://coveralls.io/r/nicoddemus/pytest-mock
-  
+
+
 Usage
 =====