Update changelog

Author: olgabrani

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add info about compatiblity in README.
 - Enable/disable cookie banner functionality via setting in config file.
 
+### Changed
+- Update instructions on module config file usage.
 
 ## [v2.1.0](https://github.com/rciam/simplesamlphp-module-themevanilla/compare/v2.0.0...v2.1.0) - 2018-10-18