Add sentry-slack to 3rd party extensions

docs/plugins/index.rst

@@ -63,6 +63,7 @@ The following extensions are available and maintained by members of the Sentry c
 * `sentry-whatsapp <https://github.com/ecarreras/sentry-whatsapp>`_
 * `sentry-xmpp <https://github.com/chroto/sentry-xmpp>`_
 * `sentry-youtrack <https://github.com/bogdal/sentry-youtrack>`_
+* `sentry-slack <https://github.com/getsentry/sentry-slack>`_
 
 Have an extension that should be listed here? Submit a `pull request <https://github.com/getsentry/sentry>`_ and we'll
 get it added.