QtIFW

Improve documentation on how to build the Installer Framework

Make it clear that you only need to build Qt statically if you want
to compile the Installer Framework on your own.

Change-Id: I45555679197e2cb7143934e9e37ac5afcb9a71cc
Reviewed-by: Niels Weber <niels.weber@digia.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>