Update RELEASE.md and README.md for release note. (#817)

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

README.md

@@ -91,6 +91,7 @@ version of TensorFlow I/O according to the table below:
 
 | TensorFlow I/O Version | TensorFlow Compatibility | Release Date |
 | --- | --- | --- |
+| 0.12.0 | 2.1.x | Feb 28, 2020 |
 | 0.11.0 | 2.1.x | Jan 10, 2020 |
 | 0.10.0 | 2.0.x | Dec 5, 2019 |
 | 0.9.1 | 2.0.x | Nov 15, 2019 |
@@ -140,7 +141,10 @@ bazel run //tools/lint:lint -- bazel clang
 On macOS Catalina or higher, it is possible to build tensorflow-io with
 system provided python 3 (3.7.3). Both `tensorflow` and `bazel` are needed.
 
-Note there is a bug in macOS's native python 3.7.3 that could be fixed
+Note Xcode installation is needed as tensorflow-io requires Swift for accessing
+Apple's native AVFoundation APIs.
+
+Note also there is a bug in macOS's native python 3.7.3 that could be fixed
 with https://github.com/tensorflow/tensorflow/issues/33183#issuecomment-554701214
 
 ```sh

RELEASE.md

@@ -1,3 +1,24 @@
+# Release 0.12.0
+
+## Major Features
+* Add Windows support for tensorflow-io.
+* Add PostgreSQL server dataset support.
+* Add MP3 format for audio dataset support.
+* Add ArrowIOTensor with input from a pyarrow.Table.
+* Add Numpy dataset support for numpy file and memory input.
+* Add decode_avro/encode_avro for Avro serialization support.
+* Prometheus dataset now outputs structured shapes.
+
+## Thanks to our Contributors
+
+This release contains contributions from many people:
+
+Bryan Cutler, DylanTallchiefGit, Kota Yamaguchi, Mark Daoust,
+Suyash Kumar, Yong Tang, Yuan Tang
+
+We are also grateful to all who filed issues or helped resolve them, asked and
+answered questions, and were part of inspiring discussions.
+
 # Release 0.11.0
 
 ## Major Features