Include Slack in list of places to get help

As Slack is a common replacement for IRC for projects (e.g. Wordpress), I propose that we encourage the use of Slack as you have members of the core team using it while IRC often does not have any of the core team available.

CONTRIBUTING.md

@@ -19,7 +19,7 @@ and [submitting pull requests](#pull-requests), but please respect the following
 restrictions:
 
 * Please **do not** use the issue tracker for personal support requests.  Stack
-  Overflow ([`twitter-bootstrap-3`](https://stackoverflow.com/questions/tagged/twitter-bootstrap-3) tag) or [IRC](README.md#community) are better places to get help.
+  Overflow ([`twitter-bootstrap-3`](https://stackoverflow.com/questions/tagged/twitter-bootstrap-3) tag), [Slack, or IRC](README.md#community) are better places to get help.
 
 * Please **do not** derail or troll issues. Keep the discussion on topic and
   respect the opinions of others.