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]
+### Added
+- `GEMINI_MIME_STR`, the `&str` representation of the Gemini MIME
+- `Mime::new_lossy` constructor that never fails
+- "lossy" constructors for `Response` and `Status` (see `Mime::new_lossy`)
+
+### Changed
+- Some `Response` and `Status` constructors are now infallible
+
+### Deprecated
+- Instead of `gemini_mime()` use `GEMINI_MIME`
+
+## [0.2.0] - 2020-11-14
+### Added
+- Access to client certificates by [@Alch-Emi](https://github.com/Alch-Emi)