Text After Comment Form
As megadesk mentioned its hook_form_alter you're after.

Depending on what your more comfortable with you can either add this to your theme in template.php or you can add it to a module. I'm going to assume from now on that you'r putting it into template.php. An infact rather than hook_form_alter we'll use hook_form_FORM_ID_alter.

/*
* Implementation of hook_form_FORM_ID_alter
* 
* Adding text after the comment form
*/

function YOUR_MODULE_NAME_form_comment_form_alter(&amp;$form, &amp;$form_state) {

    //add an I agree to house rules checkbox to the form
    $form['markup'] = array(
                 //This is the text you want to add to the form, using the t() function allows it to be translatable
                '#markup' =&gt; t('You can leave us a message using the contact form below.'),
                //By making the weight 20 we insure that it appears at the bottom of the form
                '#weight' =&gt; 20,
        );
}


I havnt tested this but that should be what your after... Let me know if you need more.