added nix flake file

flake.nix

@@ -0,0 +1,18 @@
+{
+  description = "description";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }: let
+    system = "x86_64-linux";
+    pkgs = import nixpkgs { inherit system; };
+  in {
+    devShells.${system}.default = pkgs.mkShell {
+      buildInputs = [
+        pkgs.git
+      ];
+    };
+  };
+}