update release notes for 2.5.13
diff --git a/site/src/site/releasenotes/groovy-2.5.adoc b/site/src/site/releasenotes/groovy-2.5.adoc
index 1d95f71..36a09c4 100644
--- a/site/src/site/releasenotes/groovy-2.5.adoc
+++ b/site/src/site/releasenotes/groovy-2.5.adoc
@@ -732,3 +732,12 @@
(link:https://issues.apache.org/jira/browse/GROOVY-9424[GROOVY-9424])
for known workarounds.
+[[Groovy2.5releasenotes-Addendum2513]]
+== Addendum for 2.5.13
+
+If you are using the `SecureASTCustomizer` and relying on the exact wording of error
+messages, e.g. in tests, then you may need to tweak the wording in those tests.
+
+See
+(link:https://issues.apache.org/jira/browse/GROOVY-9594[GROOVY-9594])
+for more details.
