docs(pubsub): add emulator instructions to doc (#5652)

pubsub/doc.go

@@ -63,8 +63,8 @@ Subsciptions may be created like so:
 Messages are then consumed from a subscription via callback.
 
  err := sub.Receive(context.Background(), func(ctx context.Context, m *Message) {
- 	log.Printf("Got message: %s", m.Data)
- 	m.Ack()
+	log.Printf("Got message: %s", m.Data)
+	m.Ack()
  })
  if err != nil {
 	// Handle error.
@@ -136,5 +136,26 @@ Slow Message Processing
 For use cases where message processing exceeds 30 minutes, we recommend using
 the base client in a pull model, since long-lived streams are periodically killed
 by firewalls. See the example at https://godoc.org/cloud.google.com/go/pubsub/apiv1#example-SubscriberClient-Pull-LengthyClientProcessing
+
+Emulator
+
+To use an emulator with this library, you can set the PUBSUB_EMULATOR_HOST
+environment variable to the address at which your emulator is running. This will
+send requests to that address instead of to Cloud Pub/Sub. You can then create
+and use a client as usual:
+
+	// Set PUBSUB_EMULATOR_HOST environment variable.
+	err := os.Setenv("PUBSUB_EMULATOR_HOST", "localhost:9000")
+	if err != nil {
+		// TODO: Handle error.
+	}
+	// Create client as usual.
+	client, err := pubsub.NewClient(ctx, "my-project-id")
+	if err != nil {
+		// TODO: Handle error.
+	}
+	defer client.Close()
+
+
 */
 package pubsub // import "cloud.google.com/go/pubsub"