add rtfd badge

Author: alimanfoo

README.rst

@@ -4,6 +4,9 @@ Numcodecs
 Numcodecs is a Python package providing buffer compression and transformation 
 codecs for use in data storage and communication applications.
 
+.. image:: https://readthedocs.org/projects/numcodecs/badge/?version=latest
+    :target: http://numcodecs.readthedocs.io/en/latest/?badge=latest
+
 .. image:: https://travis-ci.org/alimanfoo/numcodecs.svg?branch=master
     :target: https://travis-ci.org/alimanfoo/numcodecs