Update CONTRIBUTING to inform about code formatting

Author: c410-f3r

guide/src/contributing/index.md

@@ -20,3 +20,7 @@ development. You may want to browse the [unpublished guide documentation] for
    as that is when WebAssembly support was introduced. [Install Node].
 
 [Install Node]: https://nodejs.org/en/
+
+## Code Formatting
+
+Although formatting rules are not mandatory, it is encouraged to run `cargo run` (`rustfmt`) with its default rules within a PR to maintain a more organized code base. If necessary, a PR with a single commit that formats the entire project is also welcome.