Add badge for docs to README

Author: jorisvandenbossche

README.rst

@@ -9,6 +9,9 @@ Up to date remote data access for pandas, works for multiple versions of pandas.
 .. image:: https://coveralls.io/repos/pydata/pandas-datareader/badge.svg?branch=master
     :target: https://coveralls.io/r/pydata/pandas-datareader
 
+.. image:: https://readthedocs.org/projects/pandas-datareader/badge/?version=latest
+    :target: https://readthedocs.org/projects/pandas-datareader/?badge=latest
+
 Installation
 ------------