Release update for v0.15.0 (#1062)

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

Author: yongtang

README.md

@@ -92,6 +92,7 @@ version of TensorFlow I/O according to the table below:
 
 | TensorFlow I/O Version | TensorFlow Compatibility | Release Date |
 | --- | --- | --- |
+| 0.15.0 | 2.3.x | Aug 03, 2020 |
 | 0.14.0 | 2.2.x | Jul 08, 2020 |
 | 0.13.0 | 2.2.x | May 10, 2020 |
 | 0.12.0 | 2.1.x | Feb 28, 2020 |

RELEASE.md

@@ -1,3 +1,30 @@
+# Release 0.15.0
+
+## Major Features
+* Add basic gstpu file system for GCS/TPU support.
+* Add null handling support for `AvroRecordDataset`.
+* Add tfio.experimental.color for colorspace conversions:
+  - `tfio.experimental.color.[rgb_to_bgr|bgr_to_rgb]`: BGR
+  - `tfio.experimental.color.[rgb_to_rgba|rgba_to_rgb]`: RGBA
+  - `tfio.experimental.color.[rgb_to_ycbcr|ycbcr_to_rgb]`: YCbCr
+  - `tfio.experimental.color.[rgb_to_ypbpr|ypbpr_to_rgb]`: YPbPr
+  - `tfio.experimental.color.[rgb_to_ydbdr|ydbdr_to_rgb]`: YDbDr
+  - `tfio.experimental.color.[rgb_to_xyz|xyz_to_rgb]`: CIE XYZ
+  - `tfio.experimental.color.[rgb_to_lab|lab_to_rgb]`: CIE LAB
+  - `tfio.experimental.color.[rgb_to_yiq|yiq_to_rgb]`: YIQ
+  - `tfio.experimental.color.[rgb_to_yuv|yuv_to_rgb]`: YUV
+  - `tfio.experimental.color.rgb_to_grayscale`: Grayscale(BT 709)
+* Add `tfio.image.encode_gif` for GIF (animated) encoding support.
+
+## Thanks to our Contributors
+
+This release contains contributions from many people:
+
+Cheng Ren, Paul Shved, Vignesh Kothapalli, Yong Tang, marioecd
+
+We are also grateful to all who filed issues or helped resolve them, asked and
+answered questions, and were part of inspiring discussions.
+
 # Release 0.14.0
 
 ## Major Features