Kind of my first thought... seems to me this could be part of README.md, or as another suggested CONTRIBUTING.md

I tend to put a lot of this type of info into the readme anyway... for more complex projects, I'll include a docs/ directory with more markdown, images as needed, etc.

For that matter, I think context hints from the likes of Dockerfile(s), compose, and .github/workflows also can serve a dual-purpose here.