TwentyFifteen with a footer text customizer control including an edit shortcode.

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' );
			?>
			<div class="footer-text">
				<?php echo get_theme_mod( 'custom_footer_text', '<a href="' . esc_url( __( 'https://wordpress.org/', 'twentyfifteen' ) ) . '">' . sprintf( __( 'Proudly powered by %s', 'twentyfifteen' ), 'WordPress' ) . '</a>' ); ?>
			</div>
		</div><!-- .site-info -->
	</footer><!-- .site-footer -->

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

<?php wp_footer(); ?>

</body>
</html>

functions.php

<?php

/**
 * Enqueue the parent style
 */
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' );

/**
 * Adds an control to set a custom footer text.
 *
 * @param WP_Customize_Manager $wp_customize The customizer manager.
 */
function twentyfifteen_footer_setting_custom_footer( $wp_customize ) {
	$wp_customize->add_setting( 'custom_footer_text', array(
		'type' => 'theme_mod',
	) );
	$wp_customize->add_control( 'custom_footer_text', array(
		'label' => __( 'Custom footer text', 'twentyfifteen-footer' ),
		'type' => 'textarea',
		'section' => 'custom_footer',
	) );
	$wp_customize->add_section( 'custom_footer', array(
		'title' => __( 'Custom footer text', 'twentyfifteen-footer' ),
		'description' => __( 'Set the footer text here', 'twentyfifteen-footer' ),
		'priority' => 160,
		'capability' => 'edit_theme_options',
	) );
	$wp_customize->get_setting( 'custom_footer_text' )->transport = 'postMessage';
	$wp_customize->selective_refresh->add_partial( 'custom_footer_text', array(
		'selector' => '.site-info .footer-text',
		'render_callback' => 'twentyfifteen_footer_partial_custom_footer_text',
	)  );
}
add_action( 'customize_register', 'twentyfifteen_footer_setting_custom_footer' );

/**
 * Callback function fot the selective refresh
 */
function twentyfifteen_footer_partial_custom_footer_text() {
	echo get_theme_mod( 'custom_footer_text' );
}

style.css

/**
Theme Name: TwentyFifteen Footer Text
Template: twentyfifteen
*/