Add a chat badge

To the new gitter community for Python-Hyper.

README.rst

@@ -10,6 +10,10 @@ Pure Python, pure state-machine WebSocket implementation
 .. image:: https://codecov.io/gh/python-hyper/wsproto/branch/master/graph/badge.svg
     :target: https://codecov.io/gh/python-hyper/wsproto
     :alt: Code coverage
+.. image:: https://img.shields.io/badge/chat-join_now-brightgreen.svg
+    :target: https://gitter.im/python-hyper/community
+    :alt: Chat community
+
 
 This repository contains a pure-Python implementation of a WebSocket protocol
 stack. It's written from the ground up to be embeddable in whatever program you