[gtk+] Update the release notes

[Matthias Clasen <matthiasc src gnome org>]

commit f0e4a405f72238af1ffe3ca1975a400f1b970777
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Oct 28 07:17:16 2013 -0400

    Update the release notes

 README.in |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/README.in b/README.in
index e64f877..f43365f 100644
--- a/README.in
+++ b/README.in
@@ -73,6 +73,15 @@ Patches should be in unified diff form. (The -up option to GNU diff)
 Even better are git-formatted patches. (Use git format-patch)
 
 
+Release notes for 3.12
+======================
+
+* GtkWidget had a hack where if opacity is 0.999 we set up an opacity
+  group when rendering the widget. This is no longer needed in 3.10,
+  and GtkStack doesn't use it anymore. It has been removed in 3.12.
+  GdStack is using it, so applications should be ported from GdStack
+  to GtkStack in 3.12.
+
 Release notes for 3.10
 ======================