create a 'favicon/' folder, create a favicon/imgs/ subdirectory and put there a favicon.ico file put in favicon/ your plugin.favicon.php file (with the contents below), save, enable from the panel, enjoy

<?php
/*
Plugin Name: FavIcon
Plugin URI: http://www.flatpress.org/
Description: Adds a favicon to FlatPress
Author: NoWhereMan
Version: 1.0
Author URI: http://www.nowhereland.it/
*/ 
 
function plugin_favicon_head() {
echo '<link rel="icon" href="' .  plugin_geturl('favicon') .'imgs/favicon.ico" />';
}
 
add_action('wp_head', 'plugin_favicon_head');
 
?>