Update README getting started instructions (#60)

* Update README getting started instructions

Closes #59

* Add unfreePredicate allowing terraform

* Fix rustfmt complaint

README.md

@@ -22,8 +22,8 @@ This will leave you with a `flake.nix` file containing some glue code for
 getting a Nickel contract out of `tf-ncl`, evaluating a Nickel configuration
 and calling Terraform. It's as easy as
 ```shell
-nix run .#terraform -- hello-tf.ncl init
-nix run .#terraform -- hello-tf.ncl apply
+nix develop -c run-terraform init
+nix develop -c run-terraform apply
 ```
 
 Without Nix it's a bit more complicated. You will need to obtain the Nickel

flake.nix

@@ -1,6 +1,6 @@
 {
   inputs = {
-    nixpkgs.url = "nixpkgs/nixos-unstable";
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
     utils.url = "github:numtide/flake-utils";
     nickel.url = "github:tweag/nickel";
     topiary.url = "github:tweag/topiary";
@@ -34,7 +34,9 @@
 
           pkgs = import inputs.nixpkgs {
             localSystem = { inherit system; };
-            config = { };
+            config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+              "terraform"
+            ];
             overlays = [
               (import inputs.rust-overlay)
             ];

tf-ncl/src/intermediate.rs

@@ -112,7 +112,7 @@ impl FieldDescriptor {
     fn push_down(self, schema: &mut HashMap<String, Attribute>) -> Option<Self> {
         let (prefix, rest) = self.split_at_first_wildcard();
         let Some(attr) = attribute_at_path(schema, prefix) else {
-            return Some(self)
+            return Some(self);
         };
         match &mut attr.type_ {
             Type::Dictionary {