diff --git a/RELEASE_NOTES b/RELEASE_NOTES
index 1454bf829709..f1065863d4b7 100644
--- a/RELEASE_NOTES
+++ b/RELEASE_NOTES
@@ -14,6 +14,12 @@ The release team should empty this file after each release.
 CHANGES
 =======
 
+[section="Changes to the Mushroom Groove overlay terrain"]
+A change to the way how the Mushroom Groove terrain works might cause the need for some map rebalancing.
+Previously the mixed fungus terrain worked always like plain Mushroom Groove, now it is handled like the forest overlays.
+Movement is taken from the worst of both, defense is taken from the best value.
+[/section]
+
 [section="Example section 1"]
 Example contents 1.
 [/section]