hooks

Remove “Built with Storefront & WooCommerce” text

// =========================================================================
// REMOVE STOREFRONT FOOTER CREDIT TEXT
// =========================================================================
function remove_sf_actions() {
    remove_action( 'storefront_footer', 'storefront_credit', 20);
}
add_action( 'init', 'remove_sf_actions' );

Create a footer.php file in your child theme and include it in to the storefront_footer hook

// =========================================================================
// CUSTOM FOOTER STOREFRONT 
// =========================================================================
function storefront_custom_footer() {
    include('inc/footer.php');
}
add_action( 'storefront_footer', 'storefront_custom_footer' );