Add a custom notice before the payment on the WooCommerce checkout

functions.php

<?php

/**
 * Add add a notice before the payment form
 * Source : https://github.com/woothemes/woocommerce/blob/master/templates/checkout/review-order.php
 */
add_action( 'woocommerce_review_order_before_payment', 'arscons_before_paying_notice' );
function arscons_before_paying_notice() {
	wc_print_notice( __( 'some custom message', 'woocommerce' ), 'notice' );
}