Add changelog

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]
+
+## [1.1.0-rc1] - 2021-01-12
+
+Initial stable release candidate.
+
+[unreleased]: https://github.com/creekorful/bathyscaphe/compare/v1.1.0-rc1...HEAD
+
+[v1.1.0-rc1]: https://github.com/creekorful/bathyscaphe/releases/tag/v1.1.0-rc1