Add changelog (#114)

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]
+
+### Changed
+
+- README updated
+- Rakefile cleaned up. You can now just run `rake` which will run specs and fix linting using `standardrb`
+- Small documentation clarification [@smnscp](https://github.com/smnscp)
+- Bump `rqrcode_core` to `~> 1.0`
+
+### Breaking Change
+
+- The dependency `rqrcode_core` has a tiny breaking change to the `to_s` public method. https://github.com/whomwah/rqrcode_core/blob/master/CHANGELOG.md#breaking-changes
+
+## [1.2.0] - 2020-12-26
+
+### Changed
+
+- README updated
+- bump dependencies
+- fix `required_ruby_version` for Ruby 3 support
+
+[unreleased]: https://github.com/whomwah/rqrcode/compare/v1.2.0...HEAD
+[1.2.0]: https://github.com/whomwah/rqrcode/compare/v1.1.1...v1.2.0