Readme Driven Development written by Tom Preston-Werner in 2010 is a timeless post about using a README to scope a project and collaborate with others.

Here are the 4 key ideas I got from the post:

1. Your README is the most important document in your codebase.
2. So write your README first.
3. A README constrains you to single file which makes it a short, light-weight and not-too-precise introduction to your code.
4. A README helps with discussion and collaboration.

See also

1. Weekly incremental value