add reminder about how feeds work

public/convert/index.html

@@ -40,6 +40,7 @@
 <p>
 <input id="username" type="text" placeholder="username"/><br><span class="hint">only letters, digits, and underscore (_) allowed</span>
 </p>
+<p>Reminder: you won't see any posts on the account until the RSS feed updates at least once!</p>
 <button onclick="submit()">Submit</button>
 <div id="out">
 </div>