docs: add changelog entry

Author: DannyvdSluijs

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Bump to minimum PHP 7.2 ([#746](https://github.com/jsonrainbow/json-schema/pull/746))
 - Replace traditional syntax array with short syntax array ([#747](https://github.com/jsonrainbow/json-schema/pull/747))
 - Increase phpstan level to 8 with baseline to swallow existing errors ([#673](https://github.com/jsonrainbow/json-schema/pull/673))
+- Add ext-json to composer.json to ensure JSON extension available  ([#759](https://github.com/jsonrainbow/json-schema/pull/759))
 
 ## [6.0.0] - 2024-07-30
 ### Added