add TODO comment for experimental and thus currently undocumented CMake option

if(PKG_KOKKOS)

  # TODO: this option needs to be documented when this works with a

  # regular release version of KOKKOS, and a version compatibility check

  # of external KOKKOS lib versus what the KOKKOS package needs is required.

  option(EXTERNAL_KOKKOS "Build against external kokkos library")

  if(EXTERNAL_KOKKOS)

    find_package(Kokkos REQUIRED)