Add unreleased section to changelog

Author: stephenykp

CHANGELOG.md

@@ -3,6 +3,11 @@ 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/)
 
+## [Unreleased]
+### Fixed
+### Added
+### Changed
+
 ## [1.15.2]
 ### Fixed
 - Fixed bug where --subset in combination with the subscriber caused errors