update the readme file

README.md

@@ -6,6 +6,11 @@ Provides a file reader to read MSH file format version 4. It supports various
 grid-creator policies. Higher-order parametrized grids can be read using a
 `LagrangeGridCreator`.
 
+## Installation Instructions
+`dune-gmsh4` 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
 Either construct a grid directly
 ```c++
@@ -38,4 +43,4 @@ int main()
   auto grid = factory.createGrid();
 }
 ```
-Note, the grid-creator itself is a grid-function.
\ No newline at end of file
+Note, the grid-creator itself is a grid-function.