Add documentation on how to disable on a per-folder basis

[kevinburke]

E.g. some projects have a .ignore file or a .claude directory or similar, that says "don't turn this on for this project."

If something like that exists it would be helpful if it was documented - alternatively, instructions on what needs to go in a cfg/init.vim in order to disable Copilot for a given repository/repositories would be helpful.