Add instructions for setting up postgres locally (for new contributors)

Some of my notes from when I set it up locally (with @BethanyG's help):

```
- Set up postgres: https://wiki.postgresql.org/wiki/First_steps
- https://gist.github.com/ibraheem4/ce5ccd3e4d7a65589ce84f2a3b7c23a3
- createuser --interactive --pwprompt
- createdb cbv3
```

I'll need to double check exactly what I did. Luckily I still have the recording of it!