Add flake.nix with devShell

Author: msgilligan

flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "Preliminary devshell support for WalletLedgerExport";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    let
+    in {
+      devShells = forEachSystem (system:
+        let
+            pkgs = import nixpkgs { inherit system; };
+         in {
+            default = pkgs.mkShell {
+              buildInputs = [
+                pkgs.jdk  # JDK 21 (We're still building with ./gradlew, so install JDK not gradle)
+              ];
+              shellHook = ''
+                echo "Welcome to WalletLedgerExport"
+              '';
+            };
+         });
+    };
+}