Added a note on the CPU compatibility requirements of the ELG cluster

I ran into an issue recently with a provider who built some ELG-compatible Docker images that worked fine for them, and worked for me when I tested them on a machine outside of the ELG cluster, but failed with "illegal instruction" errors when I attempted to integrate them in the cluster. This turned out to be because one of the binaries inside the image was compiled for a newer-generation CPU instruction set than that available on the cluster VMs. So I've added a note to the "technical requirements" section of the provider documentation spelling out the exact CPU type on which ELG images must run, this can be updated if/when newer CPU nodes become available.