examples/guestbook-go: add guestbook service creation step

In the README, create the guestbook service so the guestbook server is
reachable from outside Kubernetes.
I believe this is going to be inaccurate once services v2 lands? @thockin

One thing that will work--though we may not wish to explain this on our basic introductory material--is to visit ```https://<master-ip>/api/v1beta1/proxy/services/guestbook```-- this will proxy traffic over the service. 'redirect' can be used in place of proxy, but whether the IP it returns will work depends on your network setup.