Doc: Mention Qt Quick Dialogs in getting started tutorial

As the Qt Quick Getting Started tutorial implements its own
file dialog, add a note about what we currently provide in
Qt Quick Dialogs to avoid confusion.

Change-Id: I916729a7e8561c3093ca761b10b944ae47deb90b
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>