add flake support

flake.nix

@@ -0,0 +1,26 @@
+{
+  description = "A complete and Simple Nixos Mailserver";
+
+  inputs = {
+    utils.url = "github:numtide/flake-utils";
+    nixpkgs.url = "flake:nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, utils, nixpkgs }: {
+    nixosModules.mailserver = import ./.;
+    nixosModule = self.nixosModules.mailserver;
+  } // utils.lib.eachDefaultSystem (system: let
+    pkgs = nixpkgs.legacyPackages.${system};
+  in {
+    devShell = pkgs.mkShell {
+      buildInputs = with pkgs; [
+        (python3.withPackages (p: with p; [
+          sphinx
+          sphinx_rtd_theme
+        ]))
+        jq
+        clamav
+      ];
+    };
+  });
+}