Feedback on the Fetch spec additions

I'd like to provide some preliminary feedback on the [Fetch spec patch](https://github.com/google/brotli/blob/master/fetch-spec/shared-brotli-fetch-spec.txt).

First of all, thanks for working on Shared Brotli! I'm super excited about the possibilities :)

High level comments:
* The entire approach for the protocol would highly benefit from an explainer document detailing in high-level what are the use cases that the protocol tackles, what it looks like, how it works, etc.
* Integration of the protocol's processing with Fetch is great, but you probably also need an IETF draft to describe the headers and the mechanism. Also, Fetch implies that this protocol is only relevant to browsers, and doesn't cover other HTTP clients. Is that the case?

Practical comments:
* How does the server know what dictionaries the client has? AFAICT there's no mechanism for the client to publish available dictionaries for a certain domain. Also, depending on the use-case, publishing such a list may not be feasible.
* It's not immediately clear what's done with the SRI info. Is it just for validation purposes, or is it used for caching in some way?
* The parts about `recursive-sbr` are not clear. IIUC, that's used to update a dictionary from a previous dictionary. If so, why is `Accept-encoding: sbr` forbidden? How does the server know to provide an sbr resource in this case?