Hi There,
You have to install and activate the child theme first.
After that adding this code under functions.php file locates in your child theme folder:
function add_suf_hatom_data($content) {
	$t      = get_the_modified_time('F jS, Y');
	$author = get_the_author();
	$title  = get_the_title();
	if (is_home() || is_singular() || is_archive() ) {
	$content .= '<div class="hatom-extra" style="display:none;visibility:hidden;"><span class="entry-title">'.$title.'</span> was last modified: <span class="updated"> '.$t.'</span> by <span class="author vcard"><span class="fn">'.$author.'</span></span></div>';
	}
	return $content;
}
add_filter('the_content', 'add_suf_hatom_data');
Hope it helps 