Update CHANGELOG.md

Author: ioigoume

CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
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-## [unreleased]
+## [v3.1.1](https://github.com/rciam/simplesamlphp-module-themevanilla/compare/v3.1.0...v3.1.1) - 2021-12-01
 
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