Document how to run skipper for SSE backends

[szuecs]

As shown in #1041 (comment) and easily testable with https://github.com/SubChord/go-sse/blob/master/example/send_to_client_using_reference.go , there is an issue if -write-timeout-server is set to not equal to 0 or to a very high value.

Doc of WriteTimeout:

	// WriteTimeout is the maximum duration before timing out
	// writes of the response. It is reset whenever a new
	// request's header is read. Like ReadTimeout, it does not
	// let Handlers make decisions on a per-request basis.
	WriteTimeout time.Duration

Clients can also try to send heartbeat requests with only request headers to the same backend handler to refresh the write timeout.

Idle timeouts #1041 (comment) :

• LB in front of skipper-ingress needs to be adjusted (for example AWS LoadBalancer https://docs.aws.amazon.com/elasticloadbalancing/latest/classic/config-idle-timeout.html)
• skipper-ingress needs to have a bigger idle timeout than the LB in front to delete possible race conditions that lead to 500
• see also Skipper connection to back-end times out after 1 minute regardless of timeout settings; ALB #1748 (comment)