Improve the description of --check about exit status

[akihironitta]

I am using isort --check --diff on GitHub Actions for my project, and I would like to suggest to add a statement about unix exit status of --check option as black has.

isort -h has no statement about exit status:

  -c, --check-only, --check
                        Checks the file for unsorted / unformatted imports and
                        prints them to the command line without modifying the
                        file.

while black -h clearly describes the exit status code:

  --check                         Don't write the files back, just return the
                                  status.  Return code 0 means nothing would
                                  change.  Return code 1 means some files
                                  would be reformatted.  Return code 123 means
                                  there was an internal error.