websocket: mention the gorilla package
websocket: mention the gorilla package

Updates golang/go#18152

Change-Id: Ia3df3f9668847690e60a2af0680cf1bd66042384
Reviewed-on: https://go-review.googlesource.com/33806
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

golang/net