Maintainer role: Mention the maintainer slack channel
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 <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I1a83419344840ec4de8320c2a3600254fb34df17

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