Add a note that qmake CONFIG+= only works in a clean tree

You need to use qmake -r on a tree which already has Makefiles. But you
probably need to clean up anyway in order to get the new functionality, so
this is a good recommendation for the general case.

Change-Id: Ie0fa10621a3fe961945da10a72e7fcc632f3fc0f
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>