docs: add Rust client implementation (#4592)

client-only implementation -- it *may* add server-side support in the future.

Author: tristan-f-r

Diff for: Readme.md

@@ -21,6 +21,7 @@ Some implementations in other languages are also available:
 - [Dart](https://github.com/rikulo/socket.io-client-dart)
 - [Python](https://github.com/miguelgrinberg/python-socketio)
 - [.NET](https://github.com/doghappy/socket.io-client-csharp)
+- [Rust](https://github.com/1c3t3a/rust-socketio)
 
 Its main features are: