Document how to configure platform plugin arguments in qt.conf.

Relevant commits: qtbase: 0232fa3979e0e5d8485de94950f514bfd861d22b

Change-Id: I8e14e8d0ea0171293f1ca2fb921ea24b1bc1f867
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>