Add membership notes to user memberships GET in REST API

snippet.php

<?php 

add_filter( 'woocommerce_rest_prepare_wc_user_membership', function ( WP_REST_Response $response, $post, $request ) {

	$membership = wc_memberships_get_user_membership( $post->ID );

	if ( $membership instanceof WC_Memberships_User_Membership ) {

		$data          = $response->get_data();
		$data['notes'] = $membership->get_notes();
		$response->set_data( $data );
	}

	return $response;
}, 10, 3 );