[MERGE] Developer documentation of repository stacking

[Michael Hudson]

Martin Pool wrote:
> This adds some architectural overview information about repository stacking,
> inspired by the confusion in https://bugs.edge.launchpad.net/bzr/+bug/261315
> about how it should work on the server.

[...]

> +A repository opened from an hpss server is never stacked on the server
> +side, because this could cause complexity or security problems with the
> +server acting as a proxy for the client.  Instead, the branch on the
> +server exposes the stacked-on URL and the client can open that.

It seems to me that this means we're going to need a way to open a
stacked branch without opening the basis branch...

Cheers,
mwh