TwentyFifteen Child-Theme with an additional Footer Menu

footer.php

<?php
/**
 * The template for displaying the footer
 *
 * Contains the closing of the "site-content" div and all content after.
 *
 * @package WordPress
 * @subpackage Twenty_Fifteen
 * @since Twenty Fifteen 1.0
 */
?>

	</div><!-- .site-content -->

	<footer id="colophon" class="site-footer" role="contentinfo">
		<div class="site-info">
			<?php
				/**
				 * Fires before the Twenty Fifteen footer text for footer customization.
				 *
				 * @since Twenty Fifteen 1.0
				 */
				do_action( 'twentyfifteen_credits' );
			?>
			<?php
			// Footer navigation menu.
			wp_nav_menu( array(
				'menu_class'     => 'nav-menu-footer',
				'theme_location' => 'footer',
			) );
			?>
		</div><!-- .site-info -->
	</footer><!-- .site-footer -->

</div><!-- .site -->

<?php wp_footer(); ?>

</body>
</html>

functions.php

<?php

function twentyfifteen_parent_enqueue_styles() {
	wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}
add_action( 'wp_enqueue_scripts', 'twentyfifteen_parent_enqueue_styles' );

function twentyfifteen_footer_menu_setup() {
	register_nav_menus( array(
		'footer' => __( 'Footer Menu', 'twentysixteen-footer-menu' ),
	) );
}
add_action( 'after_setup_theme', 'twentyfifteen_footer_menu_setup' );

style.css

/**
Theme Name: TwentyFifteen with Footer Menu
Template: twentyfifteen
*/
.nav-menu-footer {
	margin: 0;
}
.nav-menu-footer li {
	display: inline-block;
	list-style-type: none;
}
.nav-menu-footer li:after {
	content: " | ";
}
.nav-menu-footer li:last-child:after {
	content: none;
}