[RDFa] Relationship between RDFa and GRDDL

Hi all,

Regrets for this morning's SWD telecon, I actually simply missed the
call (was focusing on a write-up.) As per my action from the F2F, I've
produced a document that describes the relationship between RDFa and
GRDDL. I'm cc'ing the GRDDL WG, who may have comments on this topic.

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

This is short and sweet. Please let me know if it is, as a result, too
terse.

-Ben