Tom Preston-Werner (GitHub, SemVer, Jekyll, etc.) has a great blog post about writing README files.
https://tom.preston-werner.com/2010/08/23/readme-driven-deve...
> As a byproduct of writing a Readme in order to know what you need to implement, you’ll have a very nice piece of documentation sitting in front of you. You’ll also find that it’s much easier to write this document at the beginning of the project when your excitement and motivation are at their highest. Retroactively writing a Readme is an absolute drag, and you’re sure to miss all kinds of important details when you do so.