Add pubsub to README

Fixes #258 
I would mention the preview status here more prominently, maybe:

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## Google Cloud Pub/Sub

> Google Cloud Pub/Sub is in Alpha status. As a result, it might be changed in backward-incompatible ways and is not recommended for production use. It is not subject to any SLA or deprecation policy.

Google Cloud Pub/Sub allows you to connect your services with reliable, many-to-many, asynchronous messaging hosted on Google's infrastructure. Cloud Pub/Sub automatically scales as you need it and provides a foundation for building your own robust, global services.