docs: add changelog

Author: Transcript-Fetch-Admin

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project are documented here. This project follows
+[Semantic Versioning](https://semver.org/).
+
+## [0.1.0]
+
+Initial release.
+
+- `TranscriptFetch` client with `transcripts.video`, `channel`, `playlist`, `search`, and `batch`.
+- Auto-paginating async iterators: `iterChannel`, `iterPlaylist`, `iterSearch`.
+- Typed error hierarchy mapped from the API error envelope.
+- Automatic retries on 429 and 5xx with backoff, and auto-generated idempotency keys.
+- Full TypeScript types, shipped as ESM and CommonJS.