Sometimes you have a file that is related to a project, while not actually being part of it. You'd like to keep them around, but others won't need them – e.g. some notes, a log, or a database dump.
Sure, you have a project directory – but all of it is tracked by Git. A project's tmp/ directory is usually not tracked, but by definition it is not a good place to keep things.
I suggest you keep your related files in a related-files/ directory within your project(s).
To keep this directory untracked by Git in all your projects, add it to your global .gitignore file with
echo related-files >> ~/.gitignore.