More comment in CamiTKOpenSourcePackaging.cmake

The latter is to prevent to redo the same mistakes.
The CMake package_source is a default target, that
SHOULD NOT be used to produce the CamiTK tarball
(otherwise the CamiTKVersion.h is not present, and
the version compiled from the source does not have the
right version number)

Signed-off-by: Emmanuel Promayon <Emmanuel.Promayon@imag.fr>