Write specification

[melekes]

Example: https://github.com/msgpack/msgpack/blob/master/spec.md

[jaekwon]

Maybe we need to break this into smaller tasks.

• [ ] document how byte-length types ([]byte, [x]byte, []uint8, [x]uint8, string) are encoded (same as protobuf3)

[ebuchman]

It would be amazing to split this up into a section for each type (like we had with the old wire spec of : https://github.com/tendermint/tendermint/blob/v0.19.0/docs/specification/new-spec/encoding.md)

For types that are the same as protobuf, we just state that and link to the corresponding protobuf spec. For types that aren't we explain how it works and ideally why its different. I think most of that information is already in the README.

But I did notice we don't define there how we encode time!