Docs blurb about the registry

[markd at macports.org]

I'd like to put some simple descriptive text about the MacPorts registry
in the new guide in "MacPorts Internals".  I know that a new registry is
under development, but I don't even know where the current "flat" one is. 
Where is it?  And if there isn't any docs on that (other than the API
description in the guide), if anyone could give a short description of
what MacPorts uses the registry for that would be great too, assuming
there are things beyond the obvious that it does.

Also, the new guide is rapidly approaching what I think is a rough sort of
completeness.  Another day or two and I think I'll be ready to stop making
major changes for the time being.  It isn't perfect, but I think it is
more useful than the legacy guide now, and far more accessible.  I think
we could link it at the top level of the wiki and delete the
InstallingMacPorts wiki entry any time.

Mark