deploy_nixos: document closure copy concurrency support

Author: denbeigh2000

deploy_nixos/README.md

@@ -107,6 +107,7 @@ see also:
 | config\_pwd | Directory to evaluate the configuration in. This argument is required if 'config' is given | `string` | `""` | no |
 | extra\_build\_args | List of arguments to pass to the nix builder | `list(string)` | `[]` | no |
 | extra\_eval\_args | List of arguments to pass to the nix evaluation | `list(string)` | `[]` | no |
+| closure\_copy\_concurrency | Concurrency used when transferring derivations to the remote host | `number` | `1` | no |
 | hermetic | Treat the provided nixos configuration as a hermetic expression and do not evaluate using the ambient system nixpkgs. Useful if you customize eval-modules or use a pinned nixpkgs. | `bool` | false | no |
 | flake | Treat the provided nixos_config as the name of the NixOS configuration to use in the flake located in the current directory. Useful if you customize eval-modules or use a pinned nixpkgs. | `bool` | false | no |
 | keys | A map of filename to content to upload as secrets in /var/keys | `map(string)` | `{}` | no |