Small fixes to README.md (#1139)
Small fixes to README.md (#1139)

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
BrandonY/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,BrandonY/python-docs-samples,BrandonY/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,BrandonY/python-docs-samples