RFC 1929 describes the SOCKS5 username/password authentication method.
The proxy package implements this method, but doesn't mention RFC 1929.
This change adds mention of RFC 1929.

Change-Id: I30487fb41c1baa16b6ee8a99210168a597e5cb65
Reviewed-on: https://go-review.googlesource.com/60870Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>
Reviewed-by: Avelino <t@avelino.xxx>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>