Small fixes to README.md (#871)
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
GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples