add a shortcuts help

lib/locales/en-US/translation.json

@@ -200,6 +200,7 @@
           "title": "Editor shortcuts",
           "Indent": "Indent",
           "Outdent": "Outdent",
+          "Save Page": "Save Page",
           "Delete Line": "Delete Line"
       }
   }

lib/locales/ja/translation.json

@@ -197,6 +197,7 @@
     "editor": {
         "Indent": "インデント",
         "Outdent": "左インデント",
+        "Save Page": "保存",
         "Delete Line": "行削除"
     }
 }

lib/views/modal/shortcuts.html

@@ -41,6 +41,10 @@
                 <th>{{ t('modal_shortcuts.editor.Outdent') }}:</th>
                 <td><span class="key key-long">Shift</span> + <span class="key key-longer">Tab</span></td>
               </tr>
+              <tr>
+                <th>{{ t('modal_shortcuts.editor.Save Page') }}:</th>
+                <td><span class="key cmd-key"></span> + <span class="key">S</span></td>
+              </tr>
               <tr>
                 <th>{{ t('modal_shortcuts.editor.Delete Line') }}:</th>
                 <td><span class="key cmd-key"></span> + <span class="key">D</span></td>
@@ -57,6 +61,9 @@
   </div><!-- /.modal-dialog -->
 
   <script>
+    /*
+     * add classes to cmd-key by OS
+     */
     var platform = navigator.platform.toLowerCase();
     var isMac = (platform.indexOf('mac') > -1);