add missing session in changelog

Author: fredszaq

CHANGELOG.md

@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
+
+### Added
  - Add `Hash` derive similar to `std`'s one, but considering generics correctly,
    and supporting custom hash functions per field or skipping fields.
    ([#532](https://github.com/JelteF/derive_more/pull/532))