I modeled it after the READMEs for boxen's puppet-* modules. But boxen-web isn't a puppet module, so I see how following that convention might not fit.

What about changing:

```markdown
## Development

We assume you already have Boxen working on your machine.
```

to

```
## Development

We assume you already have Boxen working on your machine. You also need PostgreSQL installed.
````

Which also has the advantage of being at the top of the README.