Read the function description of dired-omit-mode

If you don't know how to enable `dired-omit-mode` for every dired buffer, like me.

Read the `dired-omit-mode` function description with `C-h f dired-omit-mode`, where it said:

To enable omitting in every Dired buffer, you can put this in
your init file:

  (add-hook 'dired-mode-hook (lambda () (dired-omit-mode)))