Documentation: Describe bare-minimum interface implementations for federation

[tsmethurst]

For implementations of go-fed that have a separate client API through which they authorize users and serve notes and statuses etc, it's not necessary to implement things such as GetOutbox and AuthenticateGetInbox/Outbox on pub.CommonBehavior. It's also not necessary to implement GetInbox on pub.FederatingProtocol. However, this is not very clear in the documentation.

It would be cool if there was some documentation along the lines of 'this is the MINIMUM of what you need to federate, and for these other functions you can just return nil because you don't need them per se'.