added Unreleased tag (#694)

Author: JosiasMB

CHANGELOG.md

@@ -5,6 +5,7 @@ All notable changes to this project will be documented in this file.
 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
 ### Added
 - Add missing `originalStartTime` field to `Event` model for recurring event instances