Warn about Gerrit Trigger Plugin and stream-events in 2.7 release notes

Upgrading to 2.7 may cause the Gerrit Trigger Plugin on Jenkins to
stop working.  Add a warning in the release notes to explain that
the Jenkins user needs to have the 'Stream Events' capability.

Change-Id: Id1d5000ea1a37500dcb27dad9ef79dad5e58cf50
1 file changed