prerequisites.md

@@ -10,7 +10,7 @@
 * Optionnal but recommended package for use of pre-processor:
    * Scipy python module
 * For visualization:
-  * Python VTK module
+  * Python VTK module (VTK >= 6 is better)
   * Paraview
 * To build the sphinx documentation:
   * Latex distribution (with pdflatex utility)