[gtk+] Add a release note about window size saving

[Matthias Clasen <matthiasc src gnome org>]

commit b9a1472fc52a8478ae79d5bdb32537476aa64b0a
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Feb 15 22:17:41 2016 -0500

    Add a release note about window size saving

 README.in |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/README.in b/README.in
index 7e7e606..5f3ce4e 100644
--- a/README.in
+++ b/README.in
@@ -82,6 +82,14 @@ Release notes for 3.20
   However, all previously existing objects still derive from GtkWidget,
   so no existing code should break.
 
+* Several fixes for window sizing and placement with client-side
+  decorations may affect applications that are saving and restoring
+  window sizes. The recommended best practice for this which is known
+  to work with client-side and server-side decorations and with older
+  and newer versions of GTK+ is to use gtk_window_get_size() to save
+  and gtk_window_set_default_size() to restore the window size. See
+  https://wiki.gnome.org/HowDoI/SaveWindowState for a detailed example.
+
 Release notes for 3.18
 ======================