Add notice about deprecating OS X 10.6 (Snow Leopard)

See discussion on the Qt development mailing list:

  http://comments.gmane.org/gmane.comp.lib.qt.devel/15202



Change-Id: If151e1514f64ab08e440c7e9234df448d6f6c75e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

dist/changes-5.3.0

@@ -15,6 +15,14 @@ corresponding to tasks in the Qt Bug Tracker:
 Each of these identifiers can be entered in the bug tracker to obtain more
 information about a particular change.
 
+****************************************************************************
+*                   Platform deprecation notice                            *
+****************************************************************************
+
+ - The support for deployment on OS X 10.6 (Snow Leopard) is deprecated
+   in Qt 5.3, and will be removed in Qt 5.4. The platform is currently
+   not tested, and most likely has issues that are not fully documented.
+
 ****************************************************************************
 *                          Library                                         *
 ****************************************************************************