Document nice-to-have things when using foca for serious business

[caio]

There are some things foca doesn't do which would be nice to have them explicit.

Foca doesn't:

• Try to recover from zero-membership: if it reaches zero members implementors must decide how to handle it (announce to well known addresses, restart the application, w/e) (update: v0.17.0 can recover from many zero membership scenarios)
• Version its protocol (yet, at least), so running distinct versions at once may lead to problems (inability to decode certain messages, for example)
• Add any encryption to its payload, so it's trivial for anyone in the network to snoop in the topology and even join the cluster
• ...More?

(Ref #15)