Replace contrib/debian/ with a note on how to install on Debian.

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>
diff --git a/Makefile b/Makefile
index 764bd23..fbbfd2c 100644
--- a/Makefile
+++ b/Makefile
@@ -27,12 +27,7 @@
 # policies, either expressed or implied, of Vincent Driessen.
 #
 
-# Determine if we're inside a debian build .. 
-ifdef DEB_BUILD_ARCH
-   prefix=$(DESTDIR)/usr/
-else
-   prefix=/usr/local
-endif
+prefix=/usr/local
 
 # files that need mode 755
 EXEC_FILES=git-flow