doc: update the usage in README

Author: 110y

README.md

@@ -6,7 +6,78 @@ Cloud Run API Emulator provides application developers with a locally-running, e
 
 ## Usage
 
-TBW
+Run cloud-run-api-emulator by using docker like below:
+
+```
+$ docker run --publish 8000:8000 --detach ghcr.io/kauche/cloud-run-api-emulator:0.0.1
+```
+
+And then, you can use the emulator through a gRPC client like below:
+
+(The example below is written in Go, but you can use any language.)
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"cloud.google.com/go/run/apiv2/runpb"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+)
+
+func main() {
+	ctx := context.Background()
+
+	cc, err := grpc.DialContext(ctx, "localhost:8000", grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to connect to the emulator: %s\n", err)
+		os.Exit(1)
+	}
+	defer func() {
+		if err = cc.Close(); err != nil {
+			fmt.Fprintf(os.Stderr, "failed to close the connection: %s\n", err)
+			os.Exit(1)
+		}
+	}()
+
+	client := runpb.NewServicesClient(cc)
+
+	createReq := &runpb.CreateServiceRequest{
+		Parent:    "projects/test-project/locations/us-central1",
+		ServiceId: "my-service",
+		Service: &runpb.Service{
+			Description: "my service",
+		},
+	}
+
+	_, err = client.CreateService(ctx, createReq)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to create the service: %s\n", err)
+		os.Exit(1)
+	}
+
+	listReq := &runpb.ListServicesRequest{
+		Parent:   "projects/test-project/locations/us-central1",
+		PageSize: 5,
+	}
+
+	listRes, err := client.ListServices(ctx, listReq)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to list services: %s\n", err)
+		os.Exit(1)
+	}
+
+	// this prints `service: projects/test-project/locations/us-central1/services/my-service`
+	for _, service := range listRes.Services {
+		fmt.Printf("service: %s\n", service.Name)
+	}
+}
+
+```
 
 ## Supported Methods