Readme Driven Development, a term I first saw used by Tom Preston-Werner (GitHub founder): https://tom.preston-werner.com/2010/08/23/readme-driven-deve...

I find it to be a great method of development. The core use case (what the project is and why it exists), and the public interface are so important, and often overlooked when people start a project by writing the code first. I tend to write the README in my head naturally, before I start a project, so writing it down makes sense.

I also like to write the developer guide part of the README at the beginning (how to setup a development environment), so I ensure the project is easy to contribute to.