added “Subscription columns” in help.

doc/config.html

@@ -232,6 +232,13 @@ Welcome title = <img src="welcome.jpg"><p><font size=5 color=whit
       <code><b>elogd</b></code> can be used.
    </li>
    <li>
+      <b><code>Subscription columns = &lt;number&gt;</code></b><br>
+      The config page shows a list of all logbooks one can subscribe to.
+      When the list is very long it might be recommended to organize it in
+      columns. This option allows to specify the numbers of columns to use
+      (default is 1 column: a simple list).
+   </li>
+<li>
       <b><code>Guest Selection page = &lt;file&gt;</code></b><br>
       The same for installations which have a global password file. This
       means that the logbook selection page is also password protected. It
@@ -571,8 +578,7 @@ not wanted, it can be disabled by setting <code><b>Show top groups =
       <pre>
   &lt;link rel="stylesheet" type="text/css" href="default.css" media="screen"&gt;
   &lt;link rel="stylesheet" type="text/css" href="print.css" media="print"&gt;
-        </pre>
-
+      </pre>
    </li>
    <li>
       <b><code>Title image = &lt;string&gt;</code></b><br>