Adds a section to the ACF field groups page fora notes section

acf_add_notes.php

<?php
// Add additional setting option called "Notes"
add_action('acf/render_field_group_settings', 'my_acf_add_field_group_notes');
function my_acf_add_field_group_notes($field_group){
    acf_render_field_wrap(array(
        'label'			=> __('Notes','acf'),
        'instructions'	=> __('Notes','acf'),
        'type'			=> 'textarea',
        'name'			=> 'notes',
        'prefix'		=> 'acf_field_group',
        'value'			=> ( isset($field_group['notes']) ) ? $field_group['notes'] : '',
    ));
}