Add a CHANGELOG

Author: nuttycom

CHANGELOG.md

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+# Changelog
+All notable changes to this library will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this library adheres to Rust's notion of
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- `std` feature flag; building with `--no-default-features` now enables `no_std` use.