Add Discourse badges to documentation

If Discourse is here to stay long term, it'll also get a listing on PyPI via
setup.py.

Author: mjpieters

README.rst

@@ -18,12 +18,16 @@ aiosignal
    :target: https://aiosignal.readthedocs.io/
    :alt: Latest Read The Docs
 
+.. image:: https://img.shields.io/discourse/topics?server=https%3A%2F%2Faio-libs.discourse.group%2F
+   :target: https://aio-libs.discourse.group/
+   :alt: Discourse group for io-libs
+
 .. image:: https://badges.gitter.im/Join%20Chat.svg
    :target: https://gitter.im/aio-libs/Lobby
    :alt: Chat on Gitter
 
 Introduction
-------------
+============
 
 A project to manage callbacks in `asyncio` projects.
 

docs/conf.py

@@ -160,6 +160,10 @@
                'target': 'https://badge.fury.io/py/aiosignal',
                 'height': '20',
                 'alt': 'Latest PyPI package version'},
+               {'image': 'https://img.shields.io/discourse/topics?server=https%3A%2F%2Faio-libs.discourse.group%2F',
+               'target': 'https://aio-libs.discourse.group/',
+                'height': '20',
+                'alt': 'Discourse group for io-libs'},
                {'image': 'https://badges.gitter.im/Join%20Chat.svg',
                 'target': 'https://gitter.im/aio-libs/Lobby',
                 'height': '20',