Updated zulip to discourse python#625 (python#630)

* Updated zulip to discourse

* Update README.rst

* Closes python#625

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>

Author: aliya-rahmani

README.rst

@@ -1,15 +1,15 @@
 The CPython Developer's Guide
 =============================
 
-|ReadTheDocs| |Zulip| |Codestyle|
+|ReadTheDocs| |Discourse| |Codestyle|
 
 .. |ReadTheDocs| image:: https://readthedocs.org/projects/cpython-devguide/badge/
    :target: https://devguide.python.org
    :alt: Documentation Status
 
-.. |Zulip| image:: https://img.shields.io/badge/zulip-join_chat-brightgreen.svg
-   :alt: Python Zulip chat
-   :target: https://python.zulipchat.com
+.. |Discourse| image:: https://img.shields.io/badge/discourse-join_chat-brightgreen.svg
+   :alt: Python Discourse chat
+   :target: https://discuss.python.org/
 
 .. |Codestyle| image:: https://img.shields.io/badge/code%20style-black-000000.svg
    :target: https://github.com/psf/black
@@ -29,4 +29,3 @@ supports the ``venv`` module, because the ``make html`` command will create a
 virtual environment and will install the ``Sphinx`` package::
 
     make html
-