Improve readme

[jcbhmr]

DO NOT MERGE this is a draft to demo what I've done/am considering

Things to note:

• Highlighting how this is a minimal implementation of **only those things defined in https://html.spec.whatwg.org/multipage/system-state.html (i.e. not the completely separate Geolocation stuff: https://www.w3.org/TR/geolocation/)
• Emphasizing that this should compose well with other polyfills (like the https://www.w3.org/TR/geolocation/ polyfill)
• We could make this a monorepo of a bunch of related navigator polyfills like WebUSB, Geolocation, Storage, pretty much anything on https://developer.mozilla.org/en-US/docs/Web/API/Navigator) OR we could split that out into separate repos
• Images are worth 1000 words 😉
• Is this a polyfill or a ponyfill? Does it auto-apply the globals, or just export a Navigator class?

[jcbhmr]

Closing this PR since this seems like something that should be done incrementally, not all at once. I also want to open discussions about some of these goal/scope ideas/changes instead of just unilaterally writing them in the readme. 😁

oh and i also want to delete my fork and do branching prs and stuff right on this repo since I've been added as a collaborator 😆