fix vtkm documentation
README.md incorrectly implies that use3 of VTKm in sensei depends on VTK, we have also used a VTKm clone outside of VTK, and need to support that use case.
README.md incorrectly implies that use3 of VTKm in sensei depends on VTK, we have also used a VTKm clone outside of VTK, and need to support that use case.