Add some usage infos.

Hey, another one !

I was trying to figure out how I would customize the Document which I attached media to, and if it was the proper way to use kotti_media, when I accidentally found this: http://kotti.readthedocs.org/en/latest/developing/default-views.html and realize that was exactly what I was looking for.

I think a stong reminder directly in kotti_media's documentation wiil save time to others !

Cheers.