Update index.md

As i had to figure this all out by myself i though we better share this info with everybody.
There is a lot of outdated info on the net and also on the cordova docu page. I would like to make a pull request there as well, but unfortunately i don't know how.
(http://cordova.apache.org/docs/en/3.0.0/cordova_file_file.md.html)