README: Add Discord badge

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 <stephen@that.guru>
Cc: Daniel Axtens <dja@axtens.net>
[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 <dja@axtens.net>