add .envrc/flake.nix for setting up automatic build environment under NixOS (#17)

flake.nix

@@ -0,0 +1,31 @@
+{
+  description = "A basic flake with a shell";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  inputs.systems.url = "github:nix-systems/default";
+  inputs.flake-utils = {
+    url = "github:numtide/flake-utils";
+    inputs.systems.follows = "systems";
+  };
+
+  outputs =
+    { nixpkgs, flake-utils, ... }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell { 
+          packages = [
+            pkgs.bashInteractive
+            pkgs.gnumake
+            pkgs.emscripten
+            pkgs.gcc-arm-embedded
+          ];
+          shellHook = ''
+            export EM_CACHE=$(pwd)/.emscripten_cache/
+          '';
+        };
+      }
+    );
+}