Add Coveralls badge to the Readme.

[ci skip]

Author: jakirkham

README.rst

@@ -11,3 +11,6 @@ more information.
 
 .. image:: https://ci.appveyor.com/api/projects/status/7d4iko59k8hpbbik?svg=true
     :target: https://ci.appveyor.com/project/alimanfoo/zarr
+
+.. image:: https://coveralls.io/repos/github/alimanfoo/zarr/badge.svg?branch=master
+    :target: https://coveralls.io/github/alimanfoo/zarr?branch=master