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>

Changes.mdown

@@ -7,7 +7,7 @@ Release date: **not yet**
 
 * Various minor bug fixes related to internal argument passing
 
-* Add package installers for the Debian and Windows platforms.
+* Add package installers for the Windows platform.
 
 0.4.1:
 -----

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

README.mdown

@@ -36,6 +36,16 @@ installer, which can be run using the following command:
 
 	$ wget --no-check-certificate -q -O - https://github.com/nvie/gitflow/raw/develop/contrib/gitflow-installer.sh | sudo bash
 
+#### Debian
+
+Users of Debian testing and unstable can use the apt-get tool to
+install gitflow from the Debian repository:
+
+    $ apt-get install git-flow
+
+For Debian stable, one can either use the git flow installer, or the
+Debian package from unstable (it works just fine on stable too).
+
 ### Windows
 
 For Windows users, [msysgit](http://code.google.com/p/msysgit/) is a good