[xri] model/motivations document

Please find attached a first draft document I've been working on which tries
to address the motivations and model of the world we are working with in the
XRI TC.

This is about 9 pages, and not yet formatted as an OASIS deliverable. 

Besides getting general feedback on the message and positioning this paper
contains (and all of the wording/grammer/flow issues), I'd like to get
feedback on where it should live. I originally intended this to be an
introductory section in the requirements, but it has grown to the point of
completely overshadowing what we now have as requirements. 

Looking forward to initial comments. 

	-Gabe