add nix flake

flake.nix

@@ -0,0 +1,24 @@
+{
+  description = "go-grip - render your markdown files local";
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+  outputs = { self, nixpkgs, ... }@inputs: inputs.flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        go-grip = pkgs.buildGoModule {
+          name = "go-grip";
+          src = self;
+          vendorHash = "sha256-KNKlWm/UaweRlnXD7a0C446IecjLOSwnzKvSoJfcOdE=";
+        };
+      in
+      {
+        packages.default = go-grip;
+        devShells.default = pkgs.mkShell {
+          nativeBuildInputs = [ go-grip ];
+        };
+      }
+    );
+}