Add an example post-checkout hook

Add a shell script to rebuild the documentation and install
translations. Can be executed manually or used as a post-checkout hook
in Git.
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

upgrading/post-checkout

+#!/bin/sh
+
+( cd doc/ && make )
+( cd po/ && make && make install )