I've worked at Google for a while, and a few years ago I wanted to release an open-source project. Since the code was going on Git, it made sense to throw the design there too, so I just added a DESIGN.md to the top-level of the project. This has had a remarkably nice effect: sometimes, when people make large changes to the project, they update the design to reflect the changes!

See this lovely commit as an example: https://github.com/google/stenographer/commit/d678531a3e5a87...