add a note for linking to C++11 and Fortran libraries

INSTALL

@@ -10,9 +10,11 @@ There are three ways to use AMReX.
     Fortran modules via `./configure` followed by `make` and `make
     install`.  Type `./configure -h` to show help message.  An
     application code uses its build system to compile and link to the
-    AMReX library.  See `Tutorials/libamrex_C` for an example.  Note
-    that this approach relies the make system in `Tools/GNUMake/`.
-    See `Tools/GNUMake/README.md` for more details on the make system.
+    AMReX library.  Because AMReX uses C++11 and Fortran, the linker
+    needs to link the libraries.  See `Tutorials/libamrex_C` for an
+    example of this approach.  Note that this approach relies the make
+    system in `Tools/GNUMake/`.  See `Tools/GNUMake/README.md` for
+    more details on the make system.
 
 (3) CMake.