Doc: Add a note about syncqt in readme.txt

When downloading the source directly from a hosting service
instead of using git, users need to run syncqt to generate
the files under ./include directory.

Task-number: QTBUG-32397
Change-Id: I27d58c5a771b832bb49d53885c8d5688c7bab3d6
Reviewed-by: Andy Shaw <andy.shaw@digia.com>