Add RTD badge to README.md

Author: pferreir

README.rst

@@ -5,5 +5,9 @@ Flask-PluginEngine
     :target: https://travis-ci.org/indico/flask-pluginengine
 .. image:: https://coveralls.io/repos/indico/flask-pluginengine/badge.png
     :target: https://coveralls.io/r/indico/flask-pluginengine
+.. image:: https://readthedocs.org/projects/flask-pluginengine/badge/?version=latest
+    :target: https://flask-pluginengine.readthedocs.io/
 
-Plugin system for Flask applications
+
+A simple plugin system for Flask applications. More detailed documentation available at
+https://flask-pluginengine.readthedocs.io/.