Add zulipchat badge to readme (GH-7929)

Author: Mariatta

README.rst

@@ -13,6 +13,11 @@ This is Python version 3.8.0 alpha 0
    :alt: CPython code coverage on Codecov
    :target: https://codecov.io/gh/python/cpython
 
+.. image:: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
+   :alt: Python Zulip chat
+   :target: https://python.zulipchat.com
+  
+
 Copyright (c) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011,
 2012, 2013, 2014, 2015, 2016, 2017, 2018 Python Software Foundation.  All rights
 reserved.