Added overview page for Qt's WinRT port

Change-Id: I1d16d111000b74bf526546cda5ecf2377b5ac9d3
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>