add lib.simpleFlake (#5)

examples/simple-flake/flake.nix

@@ -0,0 +1,13 @@
+{
+  description = "Flake utils demo";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.simpleFlake {
+      inherit self nixpkgs;
+      name = "simple-flake";
+      overlay = ./overlay.nix;
+      shell = ./shell.nix;
+    };
+}

examples/simple-flake/overlay.nix

@@ -0,0 +1,11 @@
+final: prev:
+{
+  # this key should be the same as the simpleFlake name attribute.
+  simple-flake = {
+    # assuming that hello is a project-specific package;
+    hello = prev.hello;
+
+    # demonstrating recursive packages
+    terraform-providers = prev.terraform-providers;
+  };
+}

examples/simple-flake/shell.nix

@@ -0,0 +1,4 @@
+{ pkgs ? import <nixpkgs> }:
+pkgs.mkShell {
+  buildInputs = [ pkgs.jq ];
+}