update the readme file

README.md

@@ -106,3 +106,10 @@ for (const auto& e : elements(grid.leafGridView()))
   sphereLocalFct.unbind();
 }
 ```
+
+Installation Instructions
+=========================
+`dune-curvedgeometry` 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.
+