updated README

Author: daniel-sanche

README.rst

@@ -105,3 +105,13 @@ with the same interface as the popular `HappyBase
 <https://happybase.readthedocs.io/en/latest/>`__ library. Unlike HappyBase,
 ``google-cloud-happybase`` uses ``google-cloud-bigtable`` under the covers,
 rather than Apache HBase.
+
+``experimental async data client``
+----------------------------------
+
+`v2.23.0` includes a preview release of a new data client, accessible at `google.cloud.bigtable.data.BigtableDataClientAsync`.
+
+This new client supports asyncio, with a corresponding synchronous surface coming soon.
+
+The new client is currently in preview, and is not recommended for production use.
+