Add an example.md file to show how to deploy the client and server

EXAMPLE.md

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+# Key point of usage
+* client side must set the bootstrap content
+  * suggest to use the env config `GRPC_XDS_BOOTSTRAP_CONFIG`
+* The `target` of the client service must be `xds:///{k8s-service-name}.{service-namespace}:{port-number}`
+  * Because the name of xDS `Listener` Resource is `targetHostPortNumber := net.JoinHostPort(fullName, strconv.Itoa(int(port.Port)))`
+* The kubernetes Service Resource of the grpc server must have a name
+  * Because if no name, the name of `ClusterLoadAssignment` Resource will be `portName = fmt.Sprintf("%s.%s:%d", ep.Name, ep.Namespace, port.Port)`
+  * And it will miss-match the name of `Cluster` Resource, which is `targetHostPort := net.JoinHostPort(fullName, port.Name)`
+
+## Client Example
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  namespace: dev
+  name: greeter-client
+  labels:
+    app: greeter-grpc-client
+spec:
+  replicas: 1
+  selector:
+    matchLabels:
+      app: greeter-grpc-client
+  template:
+    metadata:
+      labels:
+        app: greeter-grpc-client
+    spec:
+      containers:
+        - name: greeter-grpc-client
+          image: greeter-grpc-client:1.0.0 # use your own grpc client service image
+          ports:
+            - containerPort: 8080 
+          env:
+            - name: GRPC_GREET_SERVER_TARGET
+              # must be xds:///{k8s-service-name}.{service-namespace}:{port-number}
+              value: "xds:///service-greeter.dev:8972"
+            - name: GRPC_XDS_BOOTSTRAP_CONFIG
+              value: "{\"xds_servers\":[{\"server_uri\":\"xds-server.default:5000\",\"channel_creds\":[{\"type\":\"insecure\"}],\"server_features\":[\"xds_v3\"]}],\"node\":{\"id\":\"client-greeter\",\"locality\":{\"zone\":\"k8s\"}}}"
+---
+apiVersion: v1
+kind: Service
+metadata:
+  namespace: dev
+  name: client-greeter
+spec:
+  selector:
+    app: greeter-grpc-client
+  ports:
+    - protocol: TCP
+      port: 8080
+      targetPort: 8080
+```
+## Server Example
+```yaml
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  namespace: dev
+  name: greeter-service
+  labels:
+    app: greeter-grpc-service
+spec:
+  replicas: 3
+  selector:
+    matchLabels:
+      app: greeter-grpc-service
+  template:
+    metadata:
+      labels:
+        app: greeter-grpc-service
+    spec:
+      containers:
+        - name: greeter-grpc-service
+          image: greeter-grpc-service:1.0.0 # use your own server image
+          ports:
+            - name: grpc
+              containerPort: 8972
+---
+apiVersion: v1
+kind: Service
+metadata:
+  namespace: dev
+  name: service-greeter
+spec:
+  selector:
+    app: greeter-grpc-service
+  ports:
+    - protocol: TCP
+      # must have the port name
+      name: grpc
+      port: 8972
+      targetPort: 'grpc'
+```
+