add_post_author.php

<?php 
/*
	Plugin Name: Autor wpisu
	Description: Dodaje informację o autorze na końcu każdego wpisu
	Author:      ziemekpr0@gmail.com
	Author URI:  https://wpadmin.pl
*/

function wpa_post_author($content) {

	$author = '<style>.wpa-post-author-info{clear:both;}.wpa-post-author-info img{float:left;margin-right:15px;}.wpa-post-author-info h5{margin-bottom:10px;}</style>';
	$author .= '<div class="wpa-post-author-info">';
		$author .= '<h5>O autorze</h5>';
		$author .= get_avatar( get_the_author_meta('user_email'), 80 );
		$author .= wpautop( '<strong>'. get_the_author_meta('display_name') .'</strong> - '. get_the_author_meta('description') );
	$author .= '</div><!-- /.wpa-post-author-info -->';

	return $content.$author;
}

add_filter( 'the_content', 'wpa_post_author' );