Record which dune-grid-glue checkout flag to use.

README

+Use of Dune-Grid-Glue
+=========================
+
+Dune-fem-fem-coupling currently works best if you checkout dune-grid-glue
+using the following commit flag:
+
+git checkout 9169a84deb4746d9b6efd13ff503e3856f843e44 
+
+
 Preparing the Sources
 =========================