Document requirements to run `spack_setup.sh`

We need to document the software needed to run spack_setup.sh:

• all requirements for spack (https://spack.readthedocs.io/en/latest/getting_started.html#system-prerequisites)
• bash > 4
• pyyaml
• perl
• curl
• gpg (only for use of local cache)
• wget
• kernel-headers on linux (knem, gdrcopy)