title: Add new XElement to Xdocument
body: <p>I have the following code, which successfully writes to an XML file. However, it overwrites each time because of the tagRegistry.Save() call being made. How can I add a new XElement to the existing file? At the moment the file is simply overwritten.</p>
  
  <pre><code>public void saveTag()
  {
      if (File.Exists("/tagRegistry.xml"))
      {
          XElement tagRegistry = XElement.Load("/tagRegistry.xml");
          XElement newTag = new XElement("Tag",
          new XElement("tag", stringUid),
          new XElement("name", desiredName),
          new XElement("latitude", latitude),
          new XElement("longitude", longitude));
          tagRegistry.Add(newTag);
  
          using (var storage = IsolatedStorageFile.GetUserStoreForApplication())
          {
              using (Stream stream = storage.CreateFile("/tagRegistry.xml"))
              {
                  tagRegistry.Save(stream);
              }
          }
  
      }
      else
      {
          XDocument tagRegistry = new XDocument(new XElement("SmartSafe"));
          tagRegistry.Element("SmartSafe").Add(new XElement("Tag",
                      new XElement("tag", stringUid),
                      new XElement("name", desiredName),
                      new XElement("latitude", latitude),
                      new XElement("longitude", longitude)));
          using (var storage = IsolatedStorageFile.GetUserStoreForApplication())
          {
              using (Stream stream = storage.CreateFile("/tagRegistry.xml"))
              {
                  tagRegistry.Save(stream);
              }
          }
      }
  }
  </code></pre>