Add nix-shell expression

shell.nix

@@ -0,0 +1,9 @@
+{ pkgs ? import <nixpkgs> {} }:
+let
+  haskellDeps = ps: with ps; [ base gloss ];
+in pkgs.mkShell {
+  buildInputs = with pkgs; [
+    haskellPackages.cabal-install
+    (haskellPackages.ghcWithPackages haskellDeps)
+  ];
+}