Doc: Update Qt licensing documentation

Add information about the license for WebP (libwebp) version 0.4.0 which
is in Qt Image Formats module.

Task-number: QTBUG-42710
Change-Id: Ia0047c59d162081b901420979240bef2c0cda962
Reviewed-by: Sami Makkonen <sami.makkonen@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>