Add headers to generated files

src/default.nix.in

@@ -1,3 +1,6 @@
+# This is a minimal `default.nix` by yarn-plugin-nixify. You can customize it
+# as needed, it will not be overwritten by the plugin.
+
 { pkgs ? import <nixpkgs> {} }:
 
 pkgs.callPackage ./yarn-project.nix {}

src/yarn-project.nix.in

@@ -1,3 +1,6 @@
+# This file is generated by running "yarn install" inside your project.
+# Manual changes might be lost - proceed with caution!
+
 { lib, bash, nodejs, perl, stdenv, writeText }:
 
 with lib;