Added Gitter badge to docs/source/index.rst

Author: aryanbhasin

docs/source/index.rst

@@ -15,6 +15,9 @@ netjsonconfig
 .. image:: https://badge.fury.io/py/netjsonconfig.svg
   :target: http://badge.fury.io/py/netjsonconfig
 
+.. image:: https://img.shields.io/gitter/room/nwjs/nw.js.svg?style=flat-square
+  :target: https://gitter.im/openwisp/general
+
 Netjsonconfig is part of the `OpenWISP project <http://openwrt.org>`_ and it's the official
 configuration engine of `OpenWISP 2 <https://github.com/openwisp/ansible-openwisp2>`_.