code: @Override
      public void addRootComment(XmlElement rootElement) {
          Map<String, Object> map = new HashMap<>();
          map.put("mgb", MergeConstants.NEW_ELEMENT_TAG);
          map.put("rootElement", rootElement);
  
          // 添加评论
          addXmlElementComment(rootElement, map, EnumNode.ADD_ROOT_COMMENT);
      }
docs: This method is called to add a comment as the first child of the root element. This method could be used to add a
  general file comment (such as a copyright notice). However, note that the XML file merge function does not deal
  with this comment. If you run the generator repeatedly, you will only retain the comment from the initial run.
  <p>
  
  The default implementation does nothing.
  
  @param rootElement
  the root element