Document existing protocol details and set up improvement process

[dhuseby]

While reworking the rust-peer to have a UI abstraction, I realized that the messages sent between universal connectivity peers are not documented anywhere other than in protobuf files. I think it is time to document the existing message formats (e.g. peer discovery, chat message, file publish, file request) and then also define a process for proposing improvements/changes and tracking the support status for each implementation.

I think we should set up a specs folder for holding the spec docs and also a specs/ucip folder for universal connectivity improvement proposal docs and tracking their status.

[dhuseby]

@p-shahi @MarcoPolo @sukunrt @achingbrain

We just talked about this is in the go-libp2p meeting. I'll back off on the formality and we'll keep it in issues and discussion threads. This issue is now to create a specs folder to contain documents that document the different messages and protocol strings used by the universal connectivity app.