Add detailed design doc with links and justifications

[nhooyr]

E.g. why functional options over options structure, why things work the way they do, the different trade offs I made.

See also google/go-cloud#908

[nhooyr]

See #54

[nhooyr]

Going with functional options over a config struct because I want the default case to be as simple and brief as possible.

[nhooyr]

This is definitely something I should do after I release the library.

[nhooyr]

Not really necessary.