But third party accessories are an essential part of any project these days, they aren't optional or afterthoughts. In that page they mention docker-compose.yml as once example. Most likely it's impossible (in its current state) to actually run that application without Docker.

Other things that would land in the folder:

- Files essential to building the project - Files essential to running the test suite - Files essential to developing on the project

IMO a "project" includes all those things. In your definition are those all "meta"? Does "meta" mean anything that's not source code?