readme: prevent newcomer pitfall when disabling nix-command

Closes #1228

README.md

@@ -81,9 +81,13 @@ Make sure to set `nixpkgs.hostPlatform` in your `configuration.nix` to either `x
 Unlike NixOS, `nix-darwin` does not have an installer, you can just run `darwin-rebuild switch` to install nix-darwin. As `darwin-rebuild` won't be installed in your `PATH` yet, you can use the following command:
 
 ```bash
-nix run nix-darwin -- switch --flake ~/.config/nix-darwin
+nix run --extra-experimental-features nix-command --extra-experimental-features flakes nix-darwin -- switch --flake ~/.config/nix-darwin
 ```
 
+Remember to back up and remove your `/etc/nix/nix.conf` beforehand. If you forget, you will be reminded.
+
+Since removing `/etc/nix/nix.conf` would disable experimintal features, you must have `--extra-experimental-features nix-command --extra-experimental-features flakes` passed as arguments to `nix run`.
+
 ### Step 3. Using `nix-darwin`
 
 After installing, you can run `darwin-rebuild` to apply changes to your system: