Add documentation on windeployqt.

Task-number: QTBUG-28766
Change-Id: I677f4056e76564fc1a497544ba46c04027ff040b
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>