What this means in practice is when we start working on anything that requires a little up front thought, architecture, or planning, the first thing we do is open up the documentation for the project and start writing. If collaboration/discussion is required, we open a pull request to facilitate discussion. However, the approach is still documented in the repo files, not in the PR, Github issue, etc.
This is very different than how most people work, but it is enjoyable and productive. It takes effort to switch as our tendency is to put too much documentation in Github issues, emails, etc. Old habits die hard and change takes time.
For this approach to be effective, documentation needs to be frictionless. Markdown is one solution – preferably in the same repo as the design/source files. Then it is easy to edit code and docs in the same editor.
Some of the benefits to this approach: