[ubl-ndrsc] Paella: Interface-Based XSD Example

Here's an updated version of the example supporting the interface-based XSD
(Paella) proposal from Barcelona. See the readme.txt for details.
Essentially this is a proposal for how to use various features of XSD to
effect specialization of UBL types (and specializations of those types...)

This is about one baby-step away from being a full-fledged position paper
IMO. About all that has to happen now is to take the readme.txt and couple
it with the pros and cons list from the minutes.

Adios,
Bill

PS For those of you with XML Spy, there's a Spy project (.spp) included.