Code below only adds the extra html to new posts.
If you don't want this extra html stored in the database, but instead only printed to screen, you should use 'the_content' filter, instead of 'content_save_pre'
To use, edit where it says 'Here is your Html', save it to a php file named add_post_content.php, put it in the plugin folder, then activate it.
Code:
<?php
/*
Plugin Name: Add Post Content
Plugin URI: Icantpostlinks
Description: Simplest Plugin EVAR
Author: Mark Fuckerberg
Version: 1.0
Author URI: Icantpostlinks
*/
function add_post_html($content)
{
return $content . '<h2>Here is Your Html</h2>';
}
add_filter('content_save_pre', 'add_post_html');
?>