Add note: how to specify user name passwd for http proxy

I tried to support specifying user name and password for HTTP proxy, and then
found there's nothing needs to be done: CFProxySupport reference mentions that
user name and password can be extracted from the information passed in.

My test confirms that `user:passwd@server_address` works for HTTP(S) proxy,
but not for SOCKS proxy.

It's good for the user to know this hidden feature.