Add CHANGELOG.md with current unreleased changes. (#371)

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
+- Alternative start codons can now be used in the `synthesis/codon` DNA -> protein translation package (#305)
+- Added a parser and writer for the `pileup` sequence alignment format (#329)
+
+### Fixed
+- `fastq` parser no longer becomes de-aligned when reading (#325)
+- `fastq` now handles optionals correctly (#323)
+
+## [0.26.0] - 2023-07-22
+Oops, we weren't keeping a changelog before this tag!
+
+[unreleased]: https://github.com/TimothyStiles/poly/compare/v0.26.0...main
+[0.26.0]: https://github.com/TimothyStiles/poly/releases/tag/v0.26.0