Add text to wordpress admin ‘Add Post’ edit

It’s not that hard, you can use the all_admin_notices-hook, which appears beneath the Post Updated/Saved, etc. messages box.

Wrapped up in a plugin:

<?php
! defined( 'ABSPATH' ) AND exit;
/** Plugin Name: (#64933) »kaiser« Add post/page note */

function wpse64933_add_posttype_note()
{
    global $post, $pagenow;

    // Abort in certain conditions, based on the global $pagenow
    if ( ! in_array(
         $pagenow
        ,array(
             'post-new.php'
            ,'post.php'
         )
    ) )
        return;

    // Abort in certain conditions, based on the global $post
    if ( ! in_array(
         $post->post_type
        ,array(
             'post'
            ,'page'
         )
    ) )
        return;

    // You can use the global $post here
    echo '<p>HELLO WORLD!</p>';
}
add_action( 'all_admin_notices', 'wpse64933_add_posttype_note' );