This license achieves everything that dual-licensing under MIT + Apache
aims for, but without the weirdness of being under two licenses.
Having checked external contributions, I feel pretty confident that I
can unilaterally make this license change, as people have only
contributed a handful of one-line changes of no significance towards
copyrighted work up to this point.
This removes mdBook in favor of a flat docs folder generating a single
`README.md` in the repository root.
Installation and usage instructions have also been expanded slightly.