title: How to add a column header below an existing column header in ag-grid for react
body: <p>Example</p>
  <pre><code>Header 1      | header 2     | header 3
  extra header  | extra header | extra header
  -------------------------------------------
  row cell 1    | row cell 2   | row cell 3
  </code></pre>
  <p>Is this the correct way to do it or is there a more elegant way?</p>
  <pre class="lang-js prettyprint-override"><code>&lt;AgGridReact&gt;
    &lt;AgGridColumn field=&quot;Header 1&quot;&gt;
      &lt;AgGridColumn field=&quot;extra header&quot; /&gt;
    &lt;/AgGridColumn &gt;
    &lt;AgGridColumn field=&quot;Header 2&quot;&gt;
      &lt;AgGridColumn field=&quot;extra header&quot; /&gt;
    &lt;/AgGridColumn&gt;
    &lt;AgGridColumn field=&quot;Header 3&quot;&gt;
      &lt;AgGridColumn field=&quot;extra header&quot; /&gt;
    &lt;/AgGridColumn&gt;
  &lt;/AgGridReact&gt;
  </code></pre>