Since gitflow is packaged for Debian and is available for testing and
sid already, remove the contrib/ directory in favour of using the
official Debian package.

Also mention how to install gitflow on Debian in the README.mdown.

Furthermore, Debian-specific things were removed from Changes.mdown
and the Makefile.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>