Tell users to delete 'demos' directory in readme

Hey,

Many thanks for enabling swishiness/swooshiness! I understand that the jquery.ui/demos folder should be deleted before your site goes live, as it poses a security risk. Unfortunately, search as I may, I can't find the reference that mentioned this. Anyways, if anyone can verify this (or just knows;), I reckon it'd be a good idea to add that to the documentation somewhere (eg readme.txt).

Cheers,

Andy