adjust README

Shamelessly stolen from Qt Creator.

Change-Id: I896208e39cafb337d5b35ea95674fd1c362d83b4
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
add modules Qt.multimedia and Qt.multimediawidgets

Task-number: QBS-340

Change-Id: I996c103d128f1c84c8e68cf4cc4ab44685f249e9
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>