docs: add changelog entry

Author: DannyvdSluijs

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix pugx badges in README ([#742](https://github.com/jsonrainbow/json-schema/pull/742))
 - Add missing property in UriResolverTest ([#743](https://github.com/jsonrainbow/json-schema/pull/743))
 - Correct casing of paths used in tests ([#745](https://github.com/jsonrainbow/json-schema/pull/745))
+- Resolve deprecations of optional parameter ([#752](https://github.com/jsonrainbow/json-schema/pull/752))
 
 ### Changed
 - Bump to minimum PHP 7.2 ([#746](https://github.com/jsonrainbow/json-schema/pull/746))