gistfile1.php

<?php
add_action('acf/render_field_group_settings', '_hwk_flexible_add_group_option', 10, 1);
function _hwk_flexible_add_group_option($group){
	acf_render_field_wrap(array(
		'label'		=> __('Ajouter au contenu flexible', 'acf'),
		'instructions'	=> __('Dossier de template par défaut', 'acf'),
		'type'		=> 'checkbox',
		'name'		=> 'hwk_flexible_content',
		'prefix'	=> 'acf_field_group',
		'value'		=> ((isset($group['hwk_flexible_content'])) ? 1 : 0),
	));
}