Improve upgrader documentation regarding protocols

Points to cover

• An application can handle subprotocol negotiation on its own by setting the Sec-Websocket-Protocol response header. The Subprotocols(r *http.Request) []string function is helpful here.
• Set u.Subprotocols for builtin subprotocol negotiation. Builtin negotiation overrides a subprotocol specified in the response header (it would have been better to return an error, but it's too late to change that).
• Builtin negotiation uses first match in u.Subprotocols. If there's no match, then no protocol is negotiated.

[marti1125]

Hi! Could I contribute with it =?

[IngCr3at1on]

Hi, @marti1125 yes absolutely! Documentation contributions are always welcome assuming they're clear and concise; my vote, feel free to open a pull request if you have documentation you believe would be useful.

[marti1125]

I am trying to understand subprotocols using client/server example

I just modify https://github.com/gorilla/websocket/blob/master/examples/echo/server.go#L23

c, err := upgrader.Upgrade(w, r, r.Header)

in the terminal show upgrade:websocket: application specific 'Sec-WebSocket-Extensions' headers are unsupported

@marti1125 It looks like you added a Sec-WebSocket-Extensions header, but that header is unrelated to subprotocols. Did you you intend to add a Sec-Websocket-Protocol instead?