Mention Slack on support documentation

docs/support.rst

@@ -11,6 +11,14 @@ Mailing list
 
 For topics and discussions that do not fit Stack Overflow's question-and-answer format, there is a Wagtail Support mailing list at `groups.google.com/d/forum/wagtail <https://groups.google.com/d/forum/wagtail>`_.
 
+Slack
+~~~~~
+
+The Wagtail Slack workspace is open to all users and developers of Wagtail. To join, head to: `https://wagtail.io/slack/ <https://wagtail.io/slack/>`_
+
+Please use the **#support** channel for support questions. Support is provided by members of the Wagtail community on a voluntary basis, and we cannot guarantee that questions will be answered quickly (or at all). If you want to see this resource succeed, please consider sticking around to help out! Also, please keep in mind that many of Wagtail's core and expert developers prefer to handle support queries on a non-realtime basis through Stack Overflow, and questions asked there may well get a better response.
+
+
 Issues
 ~~~~~~
 
@@ -19,4 +27,4 @@ If you think you've found a bug in Wagtail, or you'd like to suggest a new featu
 Torchbox
 ~~~~~~~~
 
-Finally, if you have a query which isn't relevant for either of the above forums, feel free to contact the Wagtail team at Torchbox directly, on `hello@wagtail.io <mailto:hello@wagtail.io>`_ or `@wagtailcms <http://twitter.com/wagtailcms>`_.
+Finally, if you have a query which isn't relevant for any of the above forums, feel free to contact the Wagtail team at Torchbox directly, on `hello@wagtail.io <mailto:hello@wagtail.io>`_ or `@wagtailcms <http://twitter.com/wagtailcms>`_.