Document how to set this project up for development locally?

[JoshuaKGoldberg]

I want to help! 😄

But, there's no lockfile checked in, and the .gitignore explicitly ignores a yarn.lock. Should we be using Yarn, then?

[aladdin-add]

hi, thanks! we intentioned to not using the lockfile - same reason as eslint: https://github.com/eslint/eslint#why-doesnt-eslint-lock-dependency-versions

I was using npm, but it should be fine to use other package managers like yarn/pnpm. It's nothing special to setup, just run

$ npm install
$ npm test

I'll try to put a contributing guide later. 😄