Add a note about ssh serve.

Ref: https://nix.dev/manual/nix/2.26/package-management/ssh-substituter

nix/configuration/hosts/hydra/default.nix

@@ -61,6 +61,13 @@
 
     me.image_based_appliance.enable = lib.mkForce false;
 
+    environment.systemPackages = with pkgs; [
+      htop
+    ];
+
+    # nix.sshServe.enable = true;
+    # nix.sshServe.keys = [ "ssh-dss AAAAB3NzaC1k... bob@example.org" ];
+
     me.build_in_ram.enable = true;
     me.dont_use_substituters.enable = true;
     me.minimal_base.enable = true;