skins: added new parameter "skinName" in setting.json.template

"colibris" does not exist yet, but let's mention it anyway.

settings.json.template

@@ -18,6 +18,19 @@
    */
   "favicon": "favicon.ico",
 
+  /*
+   * Skin name.
+   *
+   * Its value has to be an existing directory under src/static/skins.
+   * You can write your own, or use one of the included ones:
+   *
+   * - "no-skin":  an empty skin (default). This yields the unmodified,
+   *               traditional Etherpad theme.
+   * - "colibris": the new experimental skin (since Etherpad 1.8), candidate to
+   *               become the default in Etherpad 2.0
+   */
+  "skinName": "no-skin",
+
   /*
    * IP and port which etherpad should bind at
    */