Documentation: guidance for bootstrapping.

[zenhack]

See also #105; it would be worth writing some docs suggesting solutions to the "bootstrapping" problem -- i.e, when writing scripts to auto-generate certs, how do you automatically deal with the case where you don't already have a cert?

Per discussion on #105, it's not actually that hard, but there area a couple things that are non-obvious, so we should write it down.