Adding usual files (README, ...) that were missing

[kdenlive] / AUTHORS

Jason Wood <jasonwood@blueyonder.co.uk> : original KDE3 version author.
Gilles Caulier <caulier.gilles@free.fr> : i18n, French translations.
Jean-Baptiste Mardelle <jb@kdenlive.org> : main developer and maintainer
Marco Gittler <g.marco@freenet.de> : MLT effects, timeline, audio thumbs