Add interface for getting Hub capabilities

From Paul and Bob:

> I would love it if the spec could also
> mandate that each hub publish it's capabilities

The publishing of capabilities, as a distinct feature, is something that
has been found to be very useful in a variety of protocols -- for instance
XMPP uses this feature extensively. Older systems usually seem to have
relied on complex "handshaking" procedures that are way more complicated
than they often need to be. Just the ability to fetch some chunk of XML
that provides hub metadata is probably all that's needed here.
I know that folk will probably consider this to be "more complex" than
needed... However, if I could decide what went in the spec, I think what I
would do is find a way for a hub to publish an XRDS document that described
its capabilities and did things like carried pointers to all sorts of other
optional data (terms of service, expected downtime, biographies of the
developers, etc...)

Original issue reported on code.google.com by bslatkin on 21 Sep 2009 at 7:06

Tallefer / pubsubhubbub

Add interface for getting Hub capabilities #72