README: Add Discord badge

Add a Discord badge that can be used to connect to the server. Link
taken from daxtens' email to the list about the server [1].

[1] https://lists.ozlabs.org/pipermail/patchwork/2021-September/007226.html

Signed-off-by: Stephen Finucane <[email protected]>
Cc: Daniel Axtens <[email protected]>
[dja: change badge url to get users from the PW discord, not another
      mystery discord. Also, enable the PW discord to report its status
      via badges.]
Signed-off-by: Daniel Axtens <[email protected]>

Author: stephenfin

README.rst

@@ -18,6 +18,10 @@ Patchwork
    :target: http://patchwork.readthedocs.io/en/latest/?badge=latest
    :alt: Documentation Status
 
+.. image:: https://img.shields.io/discord/591914197219016707.svg?label=&logo=discord&logoColor=ffffff&color=7389D8&labelColor=6A7EC2
+   :target: https://discord.gg/hGWjXVTAbB
+   :alt: Discord
+
 **Patchwork** is a patch tracking system for community-based projects. It is
 intended to make the patch management process easier for both the project's
 contributors and maintainers, leaving time for the more important (and more