README.md: add pointer to Google Cloud Libraries
README.md: add pointer to Google Cloud Libraries

Fixes #203.

Change-Id: I0faab2a8fa5c1725f8d45e4a93f6f1d98ff696d9
Reviewed-on: https://code-review.googlesource.com/12971
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

google/google-api-go-client,googleapis/google-api-go-client,google/google-api-go-client,googleapis/google-api-go-client