README: add Alternatives section

Add a link to nhooyr.io/websocket package with WebAssembly support.

Updates #25

GitHub-Pull-Request: #30

Author: nhooyr

README.md

@@ -51,3 +51,8 @@ ws.AddEventListener("error", false, onError)
 err = ws.Close()
 // ...
 ```
+
+Alternatives
+------------
+
+-	[`nhooyr.io/websocket`](https://nhooyr.io/websocket) - WebSocket library with WebAssembly support.