protocol: Add deprecation note about wl_shell

Now that xdg_shell is stable and much better defined than wl_shell we
can finally deprecate wl_shell and guide users towards xdg_shell
instead.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/protocol/wayland.xml b/protocol/wayland.xml
index aabc7ae..77d991e 100644
--- a/protocol/wayland.xml
+++ b/protocol/wayland.xml
@@ -976,6 +976,9 @@
 
       It allows clients to associate a wl_shell_surface with
       a basic surface.
+
+      Note! This protocol is deprecated and not intended for production use.
+      For desktop-style user interfaces, use xdg_shell.
     </description>
 
     <enum name="error">