docs: add prerequisites for rust

README.md

@@ -85,6 +85,19 @@ If you want to get ready for `apps/desktop`, see [`apps/desktop/README.md`](apps
 
 Only needed if you're editing `rust/wasm` and want the web app to use your local build instead of the published package.
 
+**Prerequisites** — install these once before anything else:
+
+```bash
+# Rust toolchain
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+
+# build the WASM package
+cargo install wasm-pack
+
+# reruns the build on file changes, used by bun dev:wasm
+cargo install cargo-watch
+```
+
 1. Build the package once from the repo root:
 
    ```bash