Provide guidance on how a client may choose to integrate with it's HTTP cache.

[garretrieger]

This PR does a couple of things:

• Reword the extend a font subset section to have explicit inputs and outputs. Adds the HTTP fetch algorithm as an input (see: IFT Method Fallback table should have an "ambivalent client" column #63, Font extension fetching bypasses CSS font fetching definitions #69 (comment))
• Adds new section "Load a Font in a User Agent with a HTTP Cache" which provides a suggested algorithm for how to interact with and save client state into the HTTP cache. Addresses Clarify how saved client state interacts with user agent caching. #72

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[svgeesus]

LGTM