Doc: update supported platforms topic

Qt Creator built against ANGLE is not supported on Windows XP,
so XP has been dropped. Windows 8 and 8.1 have been added.
On Windows, you need DirectX 9.0.

(K)Ubuntu Linux is now supported for version 11.10 on, and
Mac OS from version 10.6 on.

Change-Id: Icf6c042670b685d9c87db95aecb8b5e8f9d2a59b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>