complete readme/help
-include requirements for Spack
-explain how we temporarily fixed the DSSAT path problem (if not already done)
-include the spack clean -a
command for installation cleaning
- address ==> Error: InstallError: OpenBLAS requires both C and Fortran compilers! [https://github.com/ComputationalRadiationPhysics/picongpu/issues/2765]
Edited by GAUTRON Romain