docs: beef up readme, add quickstart

[b5]

we're not ready to launch a full docs site yet, so let's add a bit more to the readme, and move the quickstart guide into the repo while we work toward a release

closes #208, closes #210

[b5]

Thanks for the feedback all. Ready for re-review

[b5]

@Arqu yes there's a note in the quickstart that it only works for linux & mac, line 6 of quickstart.md

[ghost]

Is iroh intended to be used as a standalone library to be embedded into larger desktop rust applications? From the updated readme I can't really tell 🤔