update README.md

README.md

@@ -28,6 +28,7 @@ as well as a few extra modules. All of them are available using git:
 * https://gitlab.dune-project.org/staging/dune-uggrid 
 * https://gitlab.dune-project.org/fufem/dune-fufem.git
 * https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git 
+* https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
 * https://gitlab.mn.tu-dresden.de/iwr/dune-gmsh4.git
 
 It is advised to checkout the current 'release' branch of these modules.