docs: Update CHANGELOG

Author: BriannaDelgado

CHANGELOG.md

@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
 - Added new language constants from January 2026 API release of 81 new languages.
+- Added support for style rules CRUD endpoints in the `DeepLClient` class:
+  `create_style_rule()`, `get_style_rule()`, `update_style_rule_name()`,
+  `update_style_rule_configured_rules()`, and `delete_style_rule()`.
+- Added support for style rule custom instruction CRUD endpoints in the
+  `DeepLClient` class: `create_style_rule_custom_instruction()`,
+  `get_style_rule_custom_instruction()`,
+  `update_style_rule_custom_instruction()`, and
+  `delete_style_rule_custom_instruction()`.
+  Please refer to the README for usage instructions.
 
 ### Changed
 - Updated formality tests to accept either formal or informal output when using default formality,