title: Write to existing xml file without replacing it's contents (XmlWriter)
body: <p>I encountered following issue,</p>
  
  <p>I first write to my xml file like this:</p>
  
  <pre><code> XmlTextWriter writer = new XmlTextWriter("course.xml", null);
   writer.Formatting = Formatting.Indented;
   writer.WriteStartDocument();
  
   writer.WriteStartElement("Course");
   writer.WriteAttributeString("title", "Examle");
   writer.WriteAttributeString("started", "true");
  
   writer.WriteEndElement();
   writer.WriteEndDocument();
   writer.Close();
  </code></pre>
  
  <p>And xml output I get is:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;Course title="Example" started="true" /&gt;
  </code></pre>
  
  <p>After that I want to write more data to this xml file so I use my code again:</p>
  
  <pre><code>XmlTextWriter writer = new XmlTextWriter("course.xml", null);
  writer.Formatting = Formatting.Indented;
  writer.WriteStartDocument();
  
  writer.WriteStartElement("Course");
         writer.StartElement("Level");
                writer.StartElement("Module");
                writer.EndElement();
         writer.EndElement();
  writer.WriteEndElement();
  
  writer.WriteEndDocument();
  writer.Close();
  </code></pre>
  
  <p>And the xml output is:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;Course&gt;
      &lt;Level&gt;
          &lt;Module&gt;
          &lt;/Module&gt;
      &lt;/Level&gt;
  &lt;/Course&gt;
  </code></pre>
  
  <p>So it replaces my original data, and all attributes in the course tag. Therefore I need a way where it doesn't replace data, but instead add it inside existing tags.</p>