Add steps to configure pinry before building.

[tylerstalder]

I followed these instructions and was able to get pinry running but couldn't create a new user from the web and had to tear down the container and read the pinry docs for the initial configuration. There may be a better way to call createsuperuser from outside the container after start that would work better for this guide, but for my case it was easier to just enable signups.

[overshard]

Very nice addition to the README, thank you for this. I often gloss over this stuff and it does make it hard for new users.