Acknowledgements, dedications, information, your email address, online journal, AIM, ICQ or YM name - put all of that there. If you paste your post-fic Author's Note into the space provided on the Submission Form, it will be automatically placed after the Read? Review! link. If you include it in the file you submit, it will be placed above the Read? Review! link and may discourage people from scrolling down to the Read? Review! link, and it still won't count towards your minimum-word-count total.
Questions? Feel free to ask here.