Include a javascript from your server to add this. That way, you can modify it in the future if you need to without having to touch your client files. Of course, this won't get you an notoriety with people browsing while having javascript disabled.
<script type="text/javascript" href="http://mysite.com/siteby.js"></script>
</body>
</html>
siteby.js
would exist on your server. It would have something similar to the following:
// Create a link
var link = document.createElement("a");
link.setAttribute("href", "http://mysite.com");
link.innerHTML = "Site by Jonathan Sampson";
// Add it to the body
document.body.appendChild(link);
Just be sure that whatever you place in this script is efficient, and quick, and that your server delivers it with haste.