Update CHANGELOG.md

Author: rgomezp

CHANGELOG.md

@@ -2,6 +2,15 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
+## [1.0.0.beta1] - 2021-10-13
+### Major Release
+**INCLUDES BREAKING CHANGES**
+This release is the code-generated Ruby client made with OpenAPI Generator.
+The release is a complete overhaul and includes a new API.
+See README for full documentation.
+
+**Full Changelog**: https://github.com/OneSignal/onesignal-ruby-client/compare/v0.3.0...1.0.0.beta1
+
 ## [0.3.0] - 2016-05-10
 ### Added
 * Models dynamically create attributes on initialization