[gtk+] Add a release note about drawingarea background

[Matthias Clasen <matthiasc src gnome org>]

commit cee39bfd4684796624aa7566cc310f74698603bf
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Feb 21 00:48:56 2016 -0500

    Add a release note about drawingarea background

 README.in |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/README.in b/README.in
index a7863bd..e15a52b 100644
--- a/README.in
+++ b/README.in
@@ -90,6 +90,11 @@ Release notes for 3.20
   and gtk_window_set_default_size() to restore the window size. See
   https://wiki.gnome.org/HowDoI/SaveWindowState for a detailed example.
 
+* GtkDrawingArea used to implicitly render the theme background before
+  calling the ::draw handler. This is no longer the case. If you rely
+  on having a theme-provided background, call gtk_render_background()
+  from your ::draw handler.
+
 Release notes for 3.18
 ======================