Atom: Cat picture use case

To motivate discussion about some of the basic needs for the Atom API, I've documented a use case that I want Atom to support: Posting a Cat Picture.This use case is primarily about simple compound text/picture entries,which I think are going to be very common.  It's complicatedenough to be interesting but it's still a basic usage.

The basic idea here is that we really want compound documents thatcontain both text and pictures without users needing to worry about thegrungy details; that (X)HTML already offers a way to organize the toplevel part of this document; and that Atom should at least provide away to create such entries in a simple way.