websocket: mention the gorilla package

Updates golang/go#18152

Change-Id: Ia3df3f9668847690e60a2af0680cf1bd66042384
Reviewed-on: https://go-review.googlesource.com/33806
Reviewed-by: Ross Light <light@google.com>

websocket/websocket.go

@@ -4,6 +4,12 @@
 
 // Package websocket implements a client and server for the WebSocket protocol
 // as specified in RFC 6455.
+//
+// This package currently lacks some features found in an alternative
+// and more actively maintained WebSocket package:
+//
+//     https://godoc.org/github.com/gorilla/websocket
+//
 package websocket // import "golang.org/x/net/websocket"
 
 import (