Add Read the Docs badge

README.rst

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 .. image:: https://badge.fury.io/py/django-zombodb.svg
     :target: https://badge.fury.io/py/django-zombodb
+    :alt: PyPI Status
 
 .. image:: https://travis-ci.org/vintasoftware/django-zombodb.svg?branch=master
     :target: https://travis-ci.org/vintasoftware/django-zombodb
+    :alt: Build Status
 
 .. image:: https://codecov.io/gh/vintasoftware/django-zombodb/branch/master/graph/badge.svg
     :target: https://codecov.io/gh/vintasoftware/django-zombodb
+    :alt: Coverage Status
+
+.. image:: https://readthedocs.org/projects/django-zombodb/badge/?version=latest
+    :target: https://django-zombodb.readthedocs.io/en/latest/?badge=latest
+    :alt: Documentation Status
 
 Easy Django integration with Elasticsearch through `ZomboDB <https://github.com/zombodb/zombodb>`_ Postgres Extension.
 Thanks to ZomboDB, **your Django models are synced with Elasticsearch after every transaction**! Searching is also very simple: you can make