README instruction on which command to run

[zmberber]

The command for starting the partitioning process with disco in the README should include the flag --experimental-features "nix-command flakes", since when doing this in the most basic of setups, for example on a standard NixOS-installation live USB, it won't work without it. I think a lot of newer people not so familiar with nix that just want to use this and have it all work might not know this.

I personally would suggest putting the following two commands or something similar in the README:

# set `DISKNAME` to the name of the appropriate disk,
# e.g. `/dev/nvme0n1` or `/dev/sda`
DISKNAME="/dev/nvme0n1"
nix --experimental-features "nix-command flakes" run github:nix-community/disko -- --mode disko /tmp/disko-config.nix --arg disks "[ \"$DISKNAME\" ]"