Update Bigtable release notes [skip ci]. (#1569)

Author: coryan

google/cloud/README.md

@@ -24,6 +24,10 @@ is available [online][doxygen-link].
 
 ### v0.2.x - TBD
 
+* Implement `google::cloud::future<T>` and `google::cloud::promise<T>` based on
+  ISO/IEC TS 19571:2016, the "C++ Extensions for Concurrency" technical
+  specification, also known as "futures with continuations". 
+
 ### v0.1.x - 2018-11
 
 * `google::cloud::optional<T>` an intentionally incomplete implementation of

google/cloud/bigtable/README.md

@@ -23,6 +23,16 @@ is available [online][doxygen-link].
 
 ### v0.4.x - TBD
 
+* More experimental asynchronous APIs, note that we expect to change all these
+  experimental APIs as described in
+  [#1543](https://github.com/GoogleCloudPlatform/google-cloud-cpp/issues/1543).
+* Most of the admin operations now have asynchronous APIs.
+* All asynchronous APIs in `noex::*` return an object through which applications
+  can request cancellation of pending requests.
+* Prototype asynchronous APIs returning a `google::cloud::future<T>`,
+  applications can attach callbacks and/or block on a
+  `google::cloud::future<T>`.
+
 ### v0.3.0 - 2018-11
 
 * Include an example that illustrates how to use OpenCensus and the Cloud