Update CHANGELOG

Author: shepherd-l

OneSignalExample/Assets/OneSignal/CHANGELOG.md

@@ -5,6 +5,13 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Changed
+- Updated included Android SDK from 5.1.20 to [5.1.21](https://github.com/OneSignal/OneSignal-Android-SDK/releases/tag/5.1.21)
+  - Fix ending an already ended session
+  - Fix permissions returned by onRequestPermissionResult when the list is empty
+  - Fix keyboard input in HTML in-app messages
+  - For full changes, see the [native release notes](https://github.com/OneSignal/OneSignal-Android-SDK/releases)
+
 ## [5.1.8]
 ### Changed
 - Updated included Android SDK from 5.1.17 to [5.1.20](https://github.com/OneSignal/OneSignal-Android-SDK/releases/tag/5.1.20)