Update release notes. (#2142)

* Update release notes.

* Address review comments.

Author: devbww

google/cloud/README.md

@@ -22,7 +22,16 @@ is available [online][doxygen-link].
 
 ## Release Notes
 
-### v0.5.x - TBD
+### v0.6.x - TBD
+
+### v0.5.x - 2019-03
+
+* **Breaking change**: Make `google::cloud::optional::operator bool()` explicit.
+* Add `google::cloud::optional` value conversions that match `std::optional`.
+* Stop using grpc's `DO_NOT_USE` enum.
+* Remove ciso646 includes to force traditional spellings.
+* Change `std::endl` -> `"\n"`.
+* Enforce formatting of `.cc` files.
 
 ### v0.4.x - 2019-02
 

google/cloud/bigtable/README.md

@@ -21,7 +21,20 @@ is available [online][doxygen-link].
 
 ## Release Notes
 
-### v0.7.x - TBD
+### v0.8.x - TBD
+
+### v0.7.x - 2019-03
+
+* **Breaking change**: Return `StatusOr<>` from `TableAdmin` and `InstanceAdmin`
+  operations to signal errors.
+* Add streaming to `(Async)BulkMutator`.
+* Implement a helper class (`MutationBatcher`) to automatically batch and manage
+  outstanding bulk mutations.
+* Add `bigtable::Cell` constructors without labels argument.
+* Implementation of `RowSet` example using discontinuous keys.
+* `List{Instances,Clusters}` return `failed_locations`.
+* First version of async `Apply` batching.
+* Keep `Apply` callbacks in `MutationData`.
 
 ### v0.6.x - 2019-02
 

google/cloud/storage/README.md

@@ -20,7 +20,15 @@ the reference guide includes a quick start guide.
 
 ## Release Notes
 
-### v0.5.x - TBD
+### v0.6.x - TBD
+
+### v0.5.x - 2019-03
+
+* Properly handle subresources in V2 signed URLs.
+* Allow specifying non-default `ServiceAccountCredentials` scope and subject.
+* Add `make install` instructions.
+* Change the storage examples to throw a `std::runtime_error` on failure.
+* Add Bucket Policy Only samples.
 
 ### v0.4.x - 2019-02