Add `shell.nix` to enable development on NixOS

shell.nix

@@ -0,0 +1,22 @@
+# Defines an environment for `nix-shell` for developing Fornjot on NixOS.
+#
+# This is not complete, and probably not the correct NixOS way to do it either.
+# It's just what works for me (@hannobraun) in combination with my local
+# environment. Pull requests to improve this (best coupled with thorough
+# explanations, because I understand very little about Nix/NixOS) welcome!
+
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.mkShell {
+  packages = with pkgs; [
+    # Used as a local build tool.
+    just
+
+    # Required by dependencies.
+    openssl
+    pkg-config
+  ];
+
+  # Otherwise `export-validator` produces an error trying to link `libstdc++`.
+  LD_LIBRARY_PATH = "${pkgs.stdenv.cc.cc.lib}/lib";
+}