Update changelog

Author: ja573

CHANGELOG.md

@@ -5,6 +5,8 @@ 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 OAI-PMH
 
 ## [[1.0.4]](https://github.com/thoth-pub/thoth/releases/tag/v1.0.4) - 2026-04-13
 ### Fixed