Document retrieval protocol in the spec

[whyrusleeping]

Description

Let's get a network protocol specification written for our current retrieval protocol (even though its entirely free). This will serve as a good base for changing in the near future, and help bring the spec and implementation into parity.

Acceptance criteria

I can implement a compliant retrieval protocol entirely by looking at the spec, without once having to touch go-filecoin.

Risks + pitfalls

Where to begin

Add this to the 'network-protocols.md' document.

[whyrusleeping]

@laser you might be able to knock this one out pretty quickly.

[laser]

Sounds good @whyrusleeping.

[whyrusleeping]

This is now complete, when we implement a more sophisticated retrieval protocol we will update the spec in tandem.