ReleaseNotes: Add section for 'ref-update' hook

Change-Id: I8ba61d84f9ac8623c63ca0c1db11df2304985830

ReleaseNotes/ReleaseNotes-2.6.txt

diff --git a/ReleaseNotes/ReleaseNotes-2.6.txt b/ReleaseNotes/ReleaseNotes-2.6.txt
index ff114c6..fb790a5 100644
--- a/ReleaseNotes/ReleaseNotes-2.6.txt
+++ b/ReleaseNotes/ReleaseNotes-2.6.txt
@@ -84,6 +84,9 @@
 * link:https://code.google.com/p/gerrit/issues/detail?id=1237[Issue 1237]:
 New `merge-failed` hook and stream event when a change cannot be submitted due to failed merge.
 
+* link:https://code.google.com/p/gerrit/issues/detail?id=925[Issue 925]:
+New `ref-update` hook run before a push is accepted by Gerrit.
+
 SSH
 ~~~
 * Descriptions are added to ssh commands.