Modifying Publish example in README to match other examples given, and (#10)

fix issue #11

Author: hannahrogers-google

README.md

@@ -122,7 +122,10 @@ 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;
 ```
@@ -135,6 +138,16 @@ 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();
@@ -284,4 +297,4 @@ Java 11 | [![Kokoro CI][kokoro-badge-image-5]][kokoro-badge-link-5]
 [troubleshooting]: https://github.com/googleapis/google-cloud-common/blob/master/troubleshooting/readme.md#troubleshooting
 [contributing]: https://github.com/googleapis/java-pubsub/blob/master/CONTRIBUTING.md
 [code-of-conduct]: https://github.com/googleapis/java-pubsub/blob/master/CODE_OF_CONDUCT.md#contributor-code-of-conduct
-[license]: https://github.com/googleapis/java-pubsub/blob/master/LICENSE
+[license]: https://github.com/googleapis/java-pubsub/blob/master/LICENSE