mention the vtk writing facilities

[[Imported from SVN: r4154]]

doc/devel/features.wml

@@ -75,6 +75,7 @@ and refers you to more detailed documentation.
         <li> Output in Data Explorer format </li>
         <li> Reading and writing in the AmiraMesh format </li>
         <li> Subsampling of high-order functions </li>
+        <li> Write grids and data in the format of the visualization toolkit (vtk) </li>
       </ul>
     </li>
     <!-- Enforcement -->