Updating README file to fix Issue #6784: Pubsub: Uncaught exception i…

…n the SynchronizationContext When using example code from the Getting Started Guide

google-cloud-clients/google-cloud-pubsub/README.md

@@ -119,7 +119,10 @@ With Pub/Sub you can publish messages to a topic. Add the following import at th
 
 ```java
 import com.google.api.core.ApiFuture;
+import com.google.api.core.ApiFutureCallback;
+import com.google.api.core.ApiFutures;
 import com.google.cloud.pubsub.v1.Publisher;
+import com.google.common.util.concurrent.MoreExecutors;
 import com.google.protobuf.ByteString;
 import com.google.pubsub.v1.PubsubMessage;
 ```
@@ -132,6 +135,16 @@ try {
   ByteString data = ByteString.copyFromUtf8("my-message");
   PubsubMessage pubsubMessage = PubsubMessage.newBuilder().setData(data).build();
   ApiFuture<String> messageIdFuture = publisher.publish(pubsubMessage);
+  ApiFutures.addCallback(messageIdFuture, new ApiFutureCallback<String>() {
+    public void onSuccess(String messageId) {
+      System.out.println("published with message id: " + messageId);
+    }
+
+    public void onFailure(Throwable t) {
+      System.out.println("failed to publish: " + t);
+    }
+  }, MoreExecutors.directExecutor());
+  //...
 } finally {
   if (publisher != null) {
     publisher.shutdown();