Add nix-shell to help test/build on NixOS (#1819)

shell.nix

@@ -0,0 +1,19 @@
+# Run `nix-shell` to be able
+# to build Grin on NixOS.
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.stdenv.mkDerivation {
+  name = "grin";
+
+  buildInputs = with pkgs; [
+    ncurses cmake clang
+  ];
+
+  shellHook = ''
+      LD_LIBRARY_PATH=${pkgs.ncurses}/lib/:$LD_LIBRARY_PATH
+      LD_LIBRARY_PATH=${pkgs.zlib}/lib/:$LD_LIBRARY_PATH
+      LIBRARY_PATH=${pkgs.zlib}/lib/:$LIBRARY_PATH
+      LD_LIBRARY_PATH=${pkgs.llvmPackages.libclang}/lib/:$LD_LIBRARY_PATH
+      CXX=${pkgs.clang}/bin/clang++
+  '';
+}