db setup FAQ+ Troubleshooting

[John Vandivier]

normally we use supa directly even for local dev as opposed to a local pg install, which i don't think is clear anywhere today

also, supa has propagation time and multiple pooling modes and multiple DB connection URLs which can be confusing if you haven't set up an instance through them before

let's add docs/SETUP.md and talk about this

[Anish Pokharel]

#227

[John Vandivier]

Thanks Anish! will review soon and merge