README: preview for bigquery
README: preview for bigquery

Change-Id: Ieaf32b376da635a7493a1f75a8085b4c1a2f8ad3
Reviewed-on: https://code-review.googlesource.com/7814
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go