add template flavour text

flake.nix

@@ -102,6 +102,13 @@
       rust = {
         path = ./templates/rust;
         description = "A simple rust binary template";
+        welcomeText = ''
+          # Rust template
+          This is a simple rust binary template.
+          To build the project run `cargo build`.
+          To run the project run `cargo run`.
+          Before running nix build, make sure to run `cargo generate-lockfile` first.
+        '';
       };
     };
   };