First review draft with Martin's comments merged

This is a pretty substantial revision of the doc.
Some old material has been deleted.
Some old material has been moved to a section "Outdated Features" at the end.
The Installation section is mostly new.
It is followed by a new "what it is and how it works" section.
Everything else has been heavily edited for (one hopes) greater clarity.

Some questions remain, look for the string "??"

In a day or so I will ask for review from the mailing list readers as well.