Partitioning docs WIP

I've been working on some docs for Constraining Exclusion & Partitioning
for some time now. Deadlines seem to be looming, or may even have
passed, so it seems sensible to submit what I have now.

It's still a WIP: The final section on queries is not yet complete, but
the overall structure and flow makes sense now.

Many thanks to Josh Berkus for providing the numbered section on
implementation process, which was the starting point I'd been looking
for to describe everything else.

Any comments welcome now... flames expected for lateness.

Best Regards, Simon Riggs