Document Experimental Features

[sysarcher]

Hi @mitchellh

Thanks for the great write-up. I just wanted to point out that flakes and nix-command are experimental features and not available in older nix installs. You might want to document that people should use nix version >= 2.15.0

Also, nix-build would require passing --extra-experimental-features nix-command --extra-experimental-features flakes (you're already doing so in your Dockerfile but I'd suggest pointing it out in the write-up too)

Thanks again for the write-up. I'll use it to spread the nix