[RDFa] Use Case Document on the Wiki

Hi all,

I've started the Use Case document on the wiki. Our goal should be to
have a version ready for WG review before the Holidays.

Go ahead and edit the use cases that you're interested in: Michael on
semantic wiki, Elias on the IBM use case, Mark if you have time on the
web cut&paste.

http://www.w3.org/2006/07/SWD/wiki/RDFa_Use_Cases_and_Requirements

I'll act as editor and coalesce things into a spec document once we've
filled in the pieces.

-Ben