Add shell.nix file to aid development

spec/shell.nix

+/* 
+ * This file is a nix expression which can be used to get an isolated
+ * development environemt.
+ *
+ * When the nix package manager is installed run 
+ *  > nix-shell ./scripts/dev-env.nix 
+ * to get a shell with the dependenies of veriT present. This was only tested
+ * on NixOS, but should work on other platforms which are supported by the Nix
+ * packagemanger (such as MacOS X) too.
+ */
+
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.stdenvNoCC.mkDerivation {
+  name = "alethe";
+
+  hardeningDisable = [ "all" ];
+  buildInputs = with pkgs; [
+    gnumake
+    entr
+    python39Packages.pygments
+	];
+}