Added changelog

Author: Michael Ruettgers

CHANGELOG.md

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+# Changelog
+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]
+
+## [2.1.1] - 2020-04-15
+### Changed
+- Fixed MQTT topic to include the whole OBIS identifier
+- Look for a reading head attached to GPIO pin D2 by default
+- Switched to official libSML repository