Document HTTP-Monitor/1.1 User-Agent Healthcheck

Improving the documentation to reflect a couple of things:

- The /healthz endpoint does not require authentication
https://github.com/cloudfoundry/gorouter/blob/master/common/http/basic_auth.go#L39

- The User-Agent header can be used to make requests against the Router's main request pipeline, and it will return the the current router status.

https://github.com/cloudfoundry/gorouter/blob/master/proxy/proxy.go#L96-L99
https://github.com/cloudfoundry/gorouter/blob/master/proxy/proxy.go#L154-L156

Personally I think the '/healthz' endpoint should be available across the main request pipeline irrespective of the route specified (so it can be used instead of a User-Agent) but for now improving documentation would help other newcomers ensure their routers are monitored from the load balancers.