add docs about running private registry with gcs

cluster/addons/registry/gcs/README.md

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+# Kube-Registry with GCS storage backend
+
+Besides local file system, docker registry also supports a number of cloud storage backends. Full list of supported backend can be found [here](https://docs.docker.com/registry/configuration/#storage). This document describes how to enable GCS for kube-registry as storage backend. 
+
+A few preparation steps are needed. 
+ 1. Create a bucket named kube-registry in GCS.
+ 1. Create a service account for GCS access and create key file in json format. Detail instruction can be found [here](https://cloud.google.com/storage/docs/authentication#service_accounts).
+
+
+### Pack Keyfile into a Secret
+
+Assuming you have downloaded the keyfile as `keyfile.json`. Create secret with the `keyfile.json`...
+```console
+$ kubectl --namespace=kube-system create secret generic gcs-key-secret --from-file=keyfile=keyfile.json
+```
+
+
+### Run Registry
+
+<!-- BEGIN MUNGE: EXAMPLE registry-gcs-rc.yaml -->
+```yaml
+apiVersion: v1
+kind: ReplicationController
+metadata:
+  name: kube-registry-v0
+  namespace: kube-system
+  labels:
+    k8s-app: kube-registry
+    version: v0
+#    kubernetes.io/cluster-service: "true"
+spec:
+  replicas: 1
+  selector:
+    k8s-app: kube-registry
+    version: v0
+  template:
+    metadata:
+      labels:
+        k8s-app: kube-registry
+        version: v0
+#        kubernetes.io/cluster-service: "true"
+    spec:
+      containers:
+      - name: registry
+        image: registry:2
+        resources:
+          # keep request = limit to keep this container in guaranteed class
+          limits:
+            cpu: 100m
+            memory: 100Mi
+          requests:
+            cpu: 100m
+            memory: 100Mi
+        env:
+        - name: REGISTRY_HTTP_ADDR
+          value: :5000
+        - name: REGISTRY_STORAGE
+          value: gcs
+        - name: REGISTRY_STORAGE_GCS_BUCKET
+          value: kube-registry
+        - name: REGISTRY_STORAGE_GCS_KEYFILE
+          value: /gcs/keyfile
+        ports:
+        - containerPort: 5000
+          name: registry
+          protocol: TCP
+        volumeMounts:
+        - name: gcs-key
+          mountPath: /gcs
+      volumes:
+      - name: gcs-key
+        secret:
+          secretName: gcs-key-secret
+```
+<!-- END MUNGE: EXAMPLE registry-gcs-rc.yaml -->
+
+
+No changes are needed for other components (kube-registry service and proxy). 
+
+
+[![Analytics](https://kubernetes-site.appspot.com/UA-36037335-10/GitHub/cluster/addons/registry/gcs/README.md?pixel)]()

cluster/addons/registry/gcs/registry-gcs-rc.yaml

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+apiVersion: v1
+kind: ReplicationController
+metadata:
+  name: kube-registry-v0
+  namespace: kube-system
+  labels:
+    k8s-app: kube-registry
+    version: v0
+#    kubernetes.io/cluster-service: "true"
+spec:
+  replicas: 1
+  selector:
+    k8s-app: kube-registry
+    version: v0
+  template:
+    metadata:
+      labels:
+        k8s-app: kube-registry
+        version: v0
+#        kubernetes.io/cluster-service: "true"
+    spec:
+      containers:
+      - name: registry
+        image: registry:2
+        resources:
+          # keep request = limit to keep this container in guaranteed class
+          limits:
+            cpu: 100m
+            memory: 100Mi
+          requests:
+            cpu: 100m
+            memory: 100Mi
+        env:
+        - name: REGISTRY_HTTP_ADDR
+          value: :5000
+        - name: REGISTRY_STORAGE
+          value: gcs
+        - name: REGISTRY_STORAGE_GCS_BUCKET
+          value: kube-registry
+        - name: REGISTRY_STORAGE_GCS_KEYFILE
+          value: /gcs/keyfile
+        ports:
+        - containerPort: 5000
+          name: registry
+          protocol: TCP
+        volumeMounts:
+        - name: gcs-key
+          mountPath: /gcs
+      volumes:
+      - name: gcs-key
+        secret:
+          secretName: gcs-key-secret