Update CHANGELOG.md

Co-authored-by: Diego Barrios Romero <eldruin@gmail.com>

Author: ryan-summers

CHANGELOG.md

@@ -5,6 +5,8 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
 ## [0.7.0] - 2023-06-21
 
 * Add blanket impls of all the traits for mutable references.