fix issue #20: add shortcut to open and to hide the menu

templates/settings/admin-form.php

@@ -213,6 +213,9 @@ $choicesSizes = [
             </select>
         </div>
 
+        <p>Use the shortcut <span class="keyboard-key">Ctrl</span>+<span class="keyboard-key">o</span> 
+         to open and to hide the side menu. Use <span class="keyboard-key">tab</span> to navigate.</p>
+
         <div>
             <label for="side-menu-opener" class="settings-hint">
                 <?php p($l->t('Display the logo')); ?>
@@ -271,6 +274,15 @@ $choicesSizes = [
         </div>
     </div>
 
+    <div class="section">
+        <h2>
+            <?php p($l->t('Tips')); ?>
+        </h2>
+
+        <p class="settings-hint">Use the shortcut <span class="keyboard-key">Ctrl</span>+<span class="keyboard-key">o</span> 
+         to open and to hide the side menu. Use <span class="keyboard-key">tab</span> to navigate.</p>
+    </div>
+
     <div class="section">
         <input type="hidden" id="side-menu-cache" name="cache" value="<?php print_unescaped($_['cache']); ?>" class="side-menu-setting">