Document how to create a user for "Testing on your own machine"

Currently, there does not seem to be any documentation on how to create a user in the dev setup. I would suggest adding the following two items before the "Run db migrations" item: 1. Create a `./config.json` with `{"db": {"dialect": "sqlite", "storage": "/tmp/gancio.sqlite"}}` (without "storage", it seems non-persistent) 2. `./server/cli.js users create admin secret-password admin`