Add rejections of implicit merges on push to breaking changes

Change 477742 introduced a switch of the default value for the
`receive.rejectImplicitMerges` configuration, which was missed in
the initial version of the release notes.

Change-Id: I9d429e9d577cb2107cb33efedbe43f7fc4612ab8

pages/site/releases/3.13.md

diff --git a/pages/site/releases/3.13.md b/pages/site/releases/3.13.md
index e8c38b0..910145f 100644
--- a/pages/site/releases/3.13.md
+++ b/pages/site/releases/3.13.md
@@ -356,6 +356,13 @@
   timeout after a minute to allow other requests to make progress instead
   of blocking a thread for a long time.
 
+* [Change 477742](https://gerrit-review.googlesource.com/c/gerrit/+/477742):
+  Rejects implicit merges on push by default.
+
+  Sites that rely on pushing implicit merges must explicitly allow this behavior
+  in [project configuration](https://gerrit-documentation.storage.googleapis.com/Documentation/3.13.8/config-project-config.html#receive.rejectImplicitMerges)
+  before upgrading, otherwise affected pushes will be rejected.
+
 ## New Features
 
 * [Change 504121](https://gerrit-review.googlesource.com/c/gerrit/+/504121):