Added Flake

Nix flake that can be launched with the command "nix develop" which
creates a development environment with the unstable version of Zig
installed. This flake is essential for NixOS users like me :P

flake.nix

@@ -0,0 +1,30 @@
+{
+  # Flake dependencies
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    zig.url = "github:mitchellh/zig-overlay";
+  };
+
+  # Flake actions/outputs
+  outputs = { flake-utils, nixpkgs, zig, ... }: flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs {
+        inherit system;
+        overlays = [
+          (_: _: {
+            zigpkgs = zig.packages.${system};
+          })
+        ];
+      };
+    in with pkgs; {
+      devShells.default = mkShell {
+        buildInputs = [ zigpkgs.master ];
+        shellHook = ''
+          # Change bash shell prompt
+          export PS1="\w "
+        '';
+      };
+    }
+  );
+}