Add Readme to changelog helper script

tests/scripts/packaging/README.md

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+Git log script to pre-generate changelog
+========================================
+
+This directory only hosts a script to pre-generate the changelog for Chamilo.
+To work properly, it requires you to download Yannick Warnier's version of
+the php-git wrapper by the witty Sebastian Bergmann (author of PHP Unit, along
+other things): `git clone https://github.com/ywarnier/git php-git`.
+
+This should give you all you need to then launch 
+`php gitlog.php > /tmp/changelog` 
+and then copy-paste it to the Chamilo changelog (it still requires a careful
+manual review, but it already saves *a lot* of time).