Add information above a custom post type listing of all posts page

You can use the admin_notices action for that:

function wpa_admin_notice() {
    $screen = get_current_screen();
    if( 'your_post_type' == $screen->post_type
        && 'edit' == $screen->base ){
        ?>
        <div class="updated">
            <p>Here is some text</p>
        </div>
        <?php
    }
}
add_action( 'admin_notices', 'wpa_admin_notice' );

Change your_post_type to whatever your custom post type slug is.
Also see get_current_screen for more info on that function.