Add READMEs to be included in the QBSP image package,
machine specific instructions moved from the online docs.

Task-number: QTBUG-72815
Change-Id: I3af024aabb393eb32d866cd925e20d4f341f0d7d
Reviewed-by: Mikko Gronoff <mikko.gronoff@qt.io>
Reviewed-by: Tarja Sundqvist <tarja.sundqvist@qt.io>