Updated explainer to latest IETF draft headers

[pmeenan]

This updates the explainer to match the latest IETF draft changes and adds a "changelog" section to the end.

Specifically:

• The Sec-Available-Dictionary request header changed to Available-Dictionary.
• The value of the Available-Dictionary request header changed to be a Structured Field Byte Sequence (base-64 encoding of the dictionary hash, surrounded by colons) instead of hex-encoded string.
• The content encoding string for brotli with a dictionary changed from sbr to br-d.
• The match field of the Use-As-Dictionary response header is now a URLPattern.
• The expiration of the dictionary now uses the cache expiration of the dictionary resource instead of a separate expires.
• The server can provide an id in the Use-As-Dictionary response header which is echoed in the Dictionary-ID request header by the client in future requests.
• The server needs to send a Content-Dictionary response header with the hash of the dictionary used when compressing a response with a dictionary (must match the Available-Dictionary from the request).
• match-dest was added to the Use-As-Dictionary response header to allow for matching on fetch destinations (e.g. match-dest="document" and have the dictionary only be used for document requests).

[pmeenan]

@horo-t FYI

Some of the changes here are committed in the IETF repo but haven't been published yet (specifically the match-dest being a list of destinations instead of a single string). It will be included in draft-03 when that is published.