Add Gitter badge to the README [ci skip] (#413)

Author: jakirkham

README.rst

@@ -6,6 +6,9 @@ chunked, N-dimensional arrays, designed for use in parallel
 computing. See the `documentation <http://zarr.readthedocs.io/>`_ for
 more information.
 
+.. image:: https://badges.gitter.im/zarr-developers/community.svg
+    :target: https://gitter.im/zarr-developers/community
+
 .. image:: https://readthedocs.org/projects/zarr/badge/?version=latest
     :target: http://zarr.readthedocs.io/en/latest/?badge=latest