dev setup docs

Hi @pui, I hope to make many meaningful contributions here.

Since I don't have commit access, I cannot tell if this can be merged automatically or not. Please let me know if this PR is germane to your needs/where it could be made better.

The goal is to allow new devs to ramp up as quickly as possible. There are still a bunch of TODOs here, as this is a WIP (so don't merge yet :)), but I figured it would be useful to at least start the discussion with a PR.