diff --git a/README.in b/README.in
index 313b697fde..b6d0b99314 100644
--- a/README.in
+++ b/README.in
@@ -37,6 +37,14 @@ Release notes for 2.20
   which are doing so may need some adjustments, since the addition of the
   message area changed the internal widget hierarchy.
 
+* GDK will now set all matching virtual modifiers in the state field of
+  key events. Depending on your X modifier map, this can lead to more bits
+  in the state field being set than before. If you are manually matching
+  key events against key/modifier combinations you should make sure that
+  you properly mask away irrelevant parts of the state field. The
+  gdk_keymap_map_virtual_modifiers() function can help with matching
+  against virtual modifiers.
+
 Release notes for 2.18
 ======================