Add changelog entry for Stats API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: piobeny

CHANGELOG.md

@@ -1,3 +1,7 @@
+## [Unreleased]
+* Add StatsApi with get, by_domains, by_categories, by_email_service_providers, by_date endpoints
+* Add api_query_params to RequestParams for automatic [] serialization of list query params
+
 ## [2.4.0] - 2025-12-04
 * Fix issue #52: Update README.md using new guideline by @Ihor-Bilous in https://github.com/mailtrap/mailtrap-python/pull/55
 * Fix issue #53: Add full usage in all examples by @Ihor-Bilous in https://github.com/mailtrap/mailtrap-python/pull/56