I think that any repository, be it open source or proprietary, must have a single README file, which must include everything a new contributor needs to know about it: the product statement, the vision, the list of features and non-functional requirements, the list of technical decisions, which are important to know. Unfortunately, this file is very often forgotten, especially in non-OSS products. If you don't have it now, it's never too late to create it, and make it right.
The video is here: https://youtu.be/1zCH0xBiCP4
yegor256 podcast