Add a synthetic test username and explanation (closes #1)

index.html

@@ -31,6 +31,7 @@ <h1>OKCupid Leak Checker</h1>
           <li><a href="https://ironholds.org/blog/when-science-goes-bad-consent-data-and-doubling-down-on-the-internet/" target="_blank">Scientific consent, data, and doubling down on the internet</a></li>
           <li><a href="https://docs.google.com/document/d/1xjSi8gFT8B2jw-O8jhXykfSusggheBl-s3ud2YBca3E/edit" target="_blank">Open letter to Aarhus University</a></li>
         </ul>
+        <p>You can test that this site works by checking the test username "example_user", which was <a href="https://github.com/ryanfb/okc-leak-checker/issues/1" target="_blank">added to the username list for this purpose, and was not included in the dump</a>.</p>
       </div>
       <div class="row">
         <form id="identifier_form">