Add comment regarding specifying the compiler for MPI

packages/cable/package.py

@@ -39,6 +39,14 @@ def cmake_args(self):
         if self.spec.satisfies("+mpi"):
             args += [
                 self.define_from_variant("CABLE_MPI", "mpi"),
+                # TODO(Sean): we should let CMake find the MPI compiler instead
+                # of specifying it explicitly. However, this requires adding
+                # find_package(MPI) to the CMakeLists.txt file. This was not
+                # done in https://github.com/CABLE-LSM/CABLE/pull/200 so that
+                # we can demonstrate binary equivalence in the executables when
+                # transitioning from a Makefile based to a CMake based build
+                # system. Once find_package(MPI) is added, this file should be
+                # updated.
                 self.define(
                     "CMAKE_Fortran_COMPILER",
                     self.spec["mpi"].mpifc,