Small fixes to README.md (#871)

1. gcloud alpha should be gcloud beta
2. --pubsub-topic should be --event-pubsub-topic
3. Updated the link to the docs
4. Added info about the Publisher role for the Pub/Sub topic

Author: marciw

iot/api-client/README.md

@@ -4,29 +4,29 @@ Google Cloud IoT Core platform.
 
 ## Quickstart
 
-1. Install the gCloud CLI as described in [the device manager guide](https://cloud.google.com/iot/docs/device_manager_guide).
+1. Install the gCloud CLI as described in [the Cloud IoT Core documentation](https://cloud.google.com/iot/docs/how-tos/getting-started#set_up_the_google_cloud_sdk_and_gcloud).
 2. Create a PubSub topic:
 
     gcloud beta pubsub topics create projects/my-iot-project/topics/device-events
 
-3. Add the special account `[email protected]` to that
+3. Add the special account `[email protected]` with the role `Publisher` to that
 PubSub topic from the [Cloud Developer Console](https://console.cloud.google.com)
 or by using the helper script in the [/scripts](./scripts) folder.
 
 4. Create a registry:
 
-    gcloud alpha iot registries create my-registry \
+    gcloud beta iot registries create my-registry \
         --project=my-iot-project \
         --region=us-central1 \
-        --pubsub-topic=projects/my-iot-project/topics/device-events
+        --event-pubsub-topic=projects/my-iot-project/topics/device-events
 
 5. Use the [`generate_keys.sh`](generate_keys.sh) script to generate your signing keys:
 
     ./generate_keys.sh
 
 6. Create a device.
 
-    gcloud alpha iot devices create my-java-device \
+    gcloud beta iot devices create my-java-device \
         --project=my-iot-project \
         --region=us-central1 \
         --registry=my-registry \