docs: add platform note to contributing guide

Clarify that instructions target x86_64 Linux and can be easily
adjusted for other platforms like Linux aarch64 or macOS.

https://claude.ai/code/session_01AH9RuArd3yn2NEGwUcZqCC

Author: claude

CONTRIBUTING.md

@@ -2,6 +2,8 @@
 
 Contributions are welcome! This guide covers how to set up a development environment, build the project, run tests, and submit changes.
 
+> **Note:** The instructions below are written for x86_64 Linux. If you are building on another platform (e.g. Linux aarch64 or macOS) the same steps apply, but you may need to adjust package manager commands and download the appropriate `protoc` binary for your architecture.
+
 ## Prerequisites
 
 ### Rust