Add doc oriented toward protocol developers

[gavin-norman-sociomantic]

All existing swarm docs are aimed at users of clients based on the library.

It would also be worth having a place to store developer documentation, for example things like https://github.com/nemanja-boric-sociomantic/swarm-1/blob/68d14d48e1aa9aba92a6018778d97df955600d44/relnotes/flush.deprecated.md.

Topics to address in documentation:

• Low level protocol: version handshake, message protocol, auth protocol.
• Request protocol.
• Common: connections.
• Common: request-on-conns.
• Client: connections, connection set.
• Client: request versioning, request types, requests' access to connections.
• Client: requests, request set.
• Client: request handlers.
• Client: user notifiers.
• Node: connections, connection set.
• Node: request versioning, requests' access to connections.
• Node: requests, request set.
• Node: request handlers.

[gavin-norman-sociomantic]

An overview of the behaviour of connections / requests / ROCs in different circumstances (e.g. #310) would also be useful.

[gavin-norman-sociomantic]

Another important thing to note somewhere: when changing the protocol in swarm, deprecated code should only be removed when it is no longer used by any dependent repo (e.g. *protos).

[gavin-norman-sociomantic]

#324 added some doc. We need to extend this.

[matthias-wende-sociomantic]

Updated the connection doc. #389