Adding feature status and prerequisite to new feature.

content/en/docs/concepts/services-networking/dns-pod-service.md

@@ -159,6 +159,12 @@ record unless `publishNotReadyAddresses=True` is set on the Service.
 
 ### Pod's setHostnameAsFQDN field {#pod-sethostnameasfqdn-field}
 
+{{< feature-state for_k8s_version="v1.19" state="alpha" >}}
+
+**Prerequisites**: The `SetHostnameAsFQDN` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
+must be enabled for the
+{{< glossary_tooltip text="API Server" term_id="kube-apiserver" >}}
+
 When a Pod is configured to have fully qualified domain name (FQDN), its hostname is the short hostname. For example, if you have a Pod with the fully qualified domain name `busybox-1.default-subdomain.my-namespace.svc.cluster-domain.example`, then by default the `hostname` command inside that Pod returns `busybox-1` and  the `hostname --fqdn` command returns the FQDN.
 
 When you set `setHostnameAsFQDN: true` in the Pod spec, the kubelet writes the Pod's FQDN into the hostname for that Pod's namespace. In this case, both `hostname` and `hostname --fqdn` return the Pod's FQDN.