Maintainer role: Mention the maintainer slack channel

The community managers agreed on trying out IM for Gerrit maintainers
via Slack. The maintainer channel on Slack is private and only
accessible by maintainers. It should be used for adhoc communication
between maintainers.

This is an experiment. We will check the activity on this channel in
some month from now to see whether it is actually used. If it stays
unused we will remove it again.

[1] https://gerritcodereview.slack.com/messages/GLMGCKMEU/

Bug: Issue 11044
Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: I1a83419344840ec4de8320c2a3600254fb34df17
diff --git a/Documentation/dev-roles.txt b/Documentation/dev-roles.txt
index 52b7854..89b1436 100644
--- a/Documentation/dev-roles.txt
+++ b/Documentation/dev-roles.txt
@@ -199,7 +199,8 @@
 * create events in the
   link:https://calendar.google.com/calendar?cid=Z29vZ2xlLmNvbV91YmIxcGxhNmlqNzg1b3FianI2MWg0dmRpc0Bncm91cC5jYWxlbmRhci5nb29nbGUuY29t[
   community calendar]
-* discuss with other maintainers on the maintainer mailing list
+* discuss with other maintainers on the private maintainers mailing
+  list and Slack channel
 
 In addition, maintainers from Google can: