Mention Roundcube logs to Postfix log file.

en_US/faq/file.locations.md

@@ -197,6 +197,8 @@ Config files:
 * Config files of plugins are placed under plugin directory. for example,
   config file of `password` plugin is `plugins/password/config.inc.php`.
 
+Roundcube is configured to log to [Postfix log](#postfix) file by default.
+
 ## SOGo Groupware
 
 * Main config file is `/etc/sogo/sogo.conf`.

html/file.locations.html

@@ -241,6 +241,7 @@ releases), it has two separate config files: <code>config/db.inc.php</code> and
   config file of <code>password</code> plugin is <code>plugins/password/config.inc.php</code>.</p>
 </li>
 </ul>
+<p>Roundcube is configured to log to <a href="#postfix">Postfix log</a> file by default.</p>
 <h2 id="sogo-groupware">SOGo Groupware</h2>
 <ul>
 <li>Main config file is <code>/etc/sogo/sogo.conf</code>.</li>