Display / set / update the readme template file.

def readme_template(readme_template_file):
    """Display / set / update the readme template file."""
    if not readme_template_file:
        click.secho(dtool_config.utils.get_readme_template_fpath(
            CONFIG_PATH,
        ))
    else:
        click.secho(dtool_config.utils.set_readme_template_fpath(
            CONFIG_PATH,
            readme_template_file
        ))