Add warning about ref-updated event change in 2.12 release notes

In change I45dc114f4, the format of the `refName` attribute was
changed, which breaks triggering of `ref-updated` events in Jenkins
when using the Gerrit Trigger Plugin older than 2.15.1.

Add a warning about this in the release notes.

Change-Id: I8ae68c6ec4570d409cffbe4d34f6383fc3d9b78f
diff --git a/ReleaseNotes/ReleaseNotes-2.12.txt b/ReleaseNotes/ReleaseNotes-2.12.txt
index 72ff927..8a412af 100644
--- a/ReleaseNotes/ReleaseNotes-2.12.txt
+++ b/ReleaseNotes/ReleaseNotes-2.12.txt
@@ -29,6 +29,15 @@
 *WARNING:* The Solr secondary index is no longer supported. With this release
 the only supported secondary index is Lucene.
 
+*WARNING:* The format of the `ref-updated` event has changed. Users of the
+link:https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger[
+Jenkins Gerrit Trigger plugin] with jobs triggering on `ref-updated` should
+upgrade to at least
+link:https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger#GerritTrigger-Version2.15.1%28releasedSept142015%29[
+version 2.15.1]. If an upgrade of the plugin is not possible, a workaround is
+to change the branch configuration to type `Path` with a pattern like
+`refs/*/master` instead of `Plain` and `master`.
+
 
 Release Highlights
 ------------------
@@ -456,12 +465,20 @@
 download-commands plugin: Fix clone with commit-msg hook when project name
 contains '/'.
 
-* Include full ref names in `ref-updated` events.
+* Use full ref name in `refName` attribute of `ref-updated` events.
 +
 The link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.12/json.html#refUpdate[
 refUpdate attribute] in `ref-updated` events did not include the full name
-of the ref, i.e. `master` instead of `refs/heads/master`.
-
+of the ref in the `refName` attribute, i.e. `master` was used instead of
+`refs/heads/master`.
++
+Support for the new format is added in
+link:https://wiki.jenkins-ci.org/display/JENKINS/Gerrit+Trigger#GerritTrigger-Version2.15.1%28releasedSept142015%29[
+version 2.15.1 of the Jenkins Gerrit Trigger plugin].
++
+Users who are unable to upgrade the plugin may instead change the
+trigger's branch configuration to type `Path` with a pattern like
+`refs/*/master` instead of `Plain` and `master`.
 
 Upgrades
 --------