update the readme file

README.md

@@ -6,6 +6,11 @@ Provides structured and unstructured file writers for the VTK XML File Formats
 that can be opened in the popular ParaView visualization application. Additionally
 a file reader is provided to import VTK files into Dune grid and data objects.
 
+## Installation Instructions
+`dune-vtk` requires the DUNE core modules, version 2.7 or later.
+Please see the [general instructions for building DUNE modules](https://www.dune-project.org/doc/installation)
+for detailed instructions on how to build the module.
+
 ## Usage
 The VTK writer works similar to the dune-grid `VTKWriter`. It needs to be bound
 to a GridView and then data can be added to the points or cells in the grid.