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: Iae6e340f3ca749367472ca822de2754877a4b6cf
Reviewed-by: Andy Shaw <andy.shaw@digia.com>