Document persistent key material

[oxarbitrage]

In #85 we considered to add a section on persistent key material (https://github.com/ZcashFoundation/redjubjub/pull/85/files#r634779280) as something not included in the messages RFC.

Instead we might want to document this somewhere else or in the messages RFC itself but as a later step addition. This ticket will keep track of that and whatever is resolved to do.