Update RELEASE.md for 0.26 release (#1680)

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

Author: yongtang

README.md

@@ -126,6 +126,7 @@ of releases [here](https://github.com/tensorflow/io/releases).
 
 | TensorFlow I/O Version | TensorFlow Compatibility | Release Date |
 | --- | --- | --- |
+| 0.26.0 | 2.9.x | May 17, 2022 |
 | 0.25.0 | 2.8.x | Apr 19, 2022 |
 | 0.24.0 | 2.8.x | Feb 04, 2022 |
 | 0.23.1 | 2.7.x | Dec 15, 2021 |

RELEASE.md

@@ -1,3 +1,17 @@
+# Release 0.26.0
+
+## Major Features and Bug Fixes
+* Updated arrow version to 7.0.0
+
+## Thanks to our Contributors
+
+This release contains contributions from many people:
+
+372046933, Colin, Yong 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 0.25.0
 
 ## Major Features and Bug Fixes