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>
Merge remote-tracking branch 'origin/stable' into dev

Change-Id: I7a67d831d24b298dbf48a6cac2c5f7e519a4616a