Add a section in the README about Cloud APIs

[omaray]

Given that many users will end up on this README, we need to let them know that for Google Cloud APIs, they need to visit the google-cloud-go repository instead. We can add a section towards the top in the main README that says the following:

Google Cloud Platform APIs

If you're working with Google Cloud Platform APIs such as Datastore or Pub/Sub, consider using the Cloud Client Libraries for Go instead. These are the new and idiomatic Go libraries targeted specifically at Google Cloud Platform Services.