Document GCE Ingress Interoperability

[dominikschulz]

Please document if or how it is possible to use these Certificates to configure an HTTPS or TLS GCE Loadbalancer.

[colemickens]

As a starting point... I believe one could follow this: https://github.com/kubernetes/contrib/tree/master/ingress/controllers/gce#tls since the cert/key in the Secret correspond to the expected names for a Ingress TLS secret. (though I don't think it's going to auto-update)

[kelseyhightower]

Update the docs.