Add documentation for response streaming

[JacobHenner]

Hello,

Aside from the mention in https://github.com/taoufik07/responder/pull/308, I do not see documentation regarding stream responses. It'd be useful to document this feature.

Along those lines, is there a way for the stream-decorated function to know when the client has disconnected? I'd like to include a loop within the stream function that only exits if some condition is matched, or if the client has disconnected.