[asdf-devel] Subsumed README.source-registry into the manual

[Robert Goldman]

I had to do this quite quickly, so the import was a bit rough and ready.

BTW, I think the two configuration chapters should be pushed UP in the
manual, and the object model should be pushed DOWN below them, since the
former is for advanced users, but the latter is for in-depth programmers.

The current flow is pretty awkward.

But I don't have time for that today.

cheers,
r