Document how to use glob filters to only run linters on a subset of a filetype

[9999years]

This documents the (excellent!) workaround for #660 in the README.md.

[xudyang1]

For lazy loading nvim-lint, the following config may also be helpful:

if filetype == "yaml" and string.find(path, ".github/workflows/") then
  set_linters_cwd(opts.root_patterns_by_ft.github_action)
  lint.try_lint(opts.linters_by_ft.github_action)
  return
end