add flake

flake.nix

@@ -0,0 +1,18 @@
+{
+  description = "Flake for development workflows.";
+
+  inputs = {
+    flake-utils.url = "github:numtide/flake-utils";
+    rainix.url = "github:rainprotocol/rainix";
+  };
+
+  outputs = { self, flake-utils, rainix, ... }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = rainix.pkgs.${system};
+      in rec {
+        packages = rainix.packages.${system};
+        devShells = rainix.devShells.${system};
+      }
+    );
+}