README: mark bigtable as stable
Fixes
@@ -143,7 +143,7 @@ _October 3, 2017_
Google API | Status | Package
---------------------------------|--------------|-----------------------------------------------------------
[BigQuery][cloud-bigquery] | stable | [`cloud.google.com/go/bigquery`][cloud-bigquery-ref]
-[Bigtable][cloud-bigtable] | beta | [`cloud.google.com/go/bigtable`][cloud-bigtable-ref]
+[Bigtable][cloud-bigtable] | stable | [`cloud.google.com/go/bigtable`][cloud-bigtable-ref]
[Container][cloud-container] | alpha | [`cloud.google.com/go/container/apiv1`][cloud-container-ref]
[Data Loss Prevention][cloud-dlp]| alpha | [`cloud.google.com/go/dlp/apiv2beta1`][cloud-dlp-ref]
[Datastore][cloud-datastore] | stable | [`cloud.google.com/go/datastore`][cloud-datastore-ref]
