Add note about Rust version to CONTRIBUTING.md

Author: cknitt

CONTRIBUTING.md

@@ -18,7 +18,7 @@ Happy hacking!
 - [Yarn CLI](https://yarnpkg.com/getting-started/install) (can be installed with `corepack`, Homebrew, etc)
 - C compiler toolchain (usually installed with `xcode` on Mac)
 - Python <= 3.11 (required to build ninja)
-- Rust toolchain (required to build rewatch; follow the instructions at https://www.rust-lang.org/tools/install)
+- Rust toolchain (required to build rewatch; follow the instructions at https://www.rust-lang.org/tools/install and install the version listed as `rust-version` in `rewatch/Cargo.toml`)
 - `opam` (OCaml Package Manager) v2.2.0
 - VSCode (+ [OCaml Platform Extension](https://marketplace.visualstudio.com/items?itemName=ocamllabs.ocaml-platform))