Provide more useful instructions for building this project

TL;DR:
- include a default.nix to allow users to build an named executable
- emphasize in the README that the user needs Nix to build this project
- pin nixpkgs to a specific commit and fetch it from GitHub

scratch/brilliant/shell.nix

@@ -1,5 +1,9 @@
 let
-  pkgs = import /home/wpcarro/nixpkgs {};
+  pkgs = import (builtins.fetchGit {
+    url = "https://github.com/NixOS/nixpkgs-channels";
+    ref = "nixos-20.03";
+    rev = "afa9ca61924f05aacfe495a7ad0fd84709d236cc";
+  }) {};
 in pkgs.mkShell {
   buildInputs = with pkgs; [
     (haskellPackages.ghcWithPackages (hpkgs: with hpkgs; [