[ameliaquining]

README often contains only basic context for the project and instructions for basic tasks like running it and building it from source. If additional information for developers, like coding conventions, is short enough compared to the rest of the README then it sometimes gets added there too, but if there's a lot of it then it's frequently kept elsewhere to prevent README from getting overwhelming for end users and random people just checking out the project.

[blinkymach12]

I don't think anything requires a README.md to be monolithic. They often provide the introductory material that you mention here, then link out to other appropriate files for contribution guidelines, etc.