Added webp information in doc and changelog

Change-Id: I1287608e914598c58acff70a47f71cc0bf36f094
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>