Refactor for accessible reading

[mixmix]

I introduced @staltz to this repo the other day, and in giving him a verbal tour, I was aware that the order the details are laid on in here is really crappy for a first time person.

I think a refactor should be something like:

1. the context in which this can be used, and what it's designed for
2. how to unbox (high level), which shows how the sections/ anatomy is traversed
3. second pass with more detail
   • how the keys are derived for each section
   • a closer look at exactly how each section is composed in bytes, extra options, etc
4. links out to detailed specs + design considerations for each part