Update changelog

Author: olgabrani

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+- Add info about compatiblity in README
+
 
 ## [v2.1.0](https://github.com/rciam/simplesamlphp-module-themevanilla/compare/v2.0.0...v2.1.0) - 2018-10-18