Re: superset and subset specification

To aid momentum, I'll draft this up as a spec in markdown for discussion,
submit a pull request where it'll be a .MD proposal, than can have issues
raised against it, and be discussed on GitHub too.

Hopefully we can get 90% of the way there rapidly, then the remaining work
will be nits, process, and converting to respec if it moves forward. Along
with the obvious debate over which :Agent property to invent or use; which
may not be too difficult if webid-profile or other sets the domain of a
common property to be said :Agent, and if it's own definition states
foaf:Person is a subclass of it.

I'm familiar enough with all the related specifications, nuances around
range-14, and expected structure to get a good first stab at this.

Glad we've loosely got something to gather around that may prove to be
unblocking and backwards+forwards compatible.