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>

dist/changes-5.3.0

@@ -22,3 +22,4 @@ information about a particular change.
  - Add read/write support for Direct Draw Surface images.
  - Add read/write support for ICNS images.
  - Add read/write support for JPEG 2000 images.
+ - Add read/write support for WebP images.

src/imageformats/doc/src/qtimageformats.qdoc

@@ -59,6 +59,7 @@ libraries. If not found, it may fall back on using a bundled copy (in
 \row    \li TGA    \li Truevision Graphics Adapter      \li Read         \li No
 \row    \li TIFF   \li Tagged Image File Format         \li Read/write   \li Yes (bundled)
 \row    \li WBMP   \li Wireless Bitmap                  \li Read/write   \li No
+\row    \li WEBP   \li WebP                             \li Read/write   \li Yes (bundled)
 \endtable
 
 \section2 Deployment