The fork's documentation on the wiki, and the history left alone

Building, upstream tracking and YTSage's own README moved to the wiki; README
points there.

The history is not rewritten, unlike every other repository here. This one
still merges from oop7/YTSage and a rewrite would change every commit id, which
costs more than a clean history is worth in a fork. .github/ and
readme-translations/ are upstream's files and stay where upstream put them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -0,0 +1,23 @@
+# Changelog
+
+Kept as the work happens, not assembled at release time. Format is
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/); versions follow
+[SemVer](https://semver.org/spec/v2.0.0.html).
+
+**The `vX.Y.Z` tags already on this repository are upstream YTSage's**, inherited
+with its history. SageTube's own versions start below and are the ones this file
+records.
+
+## Unreleased
+
+### Changed
+
+- The documentation left the repository: `docs/BUILDING.md`, `docs/UPSTREAM.md`
+  and `docs/UPSTREAM_README.md` are now the
+  [wiki](https://git.houmeres.sk/Houmeres/SageTube/wiki).
+- **The history was deliberately not rewritten.** Every other repository in this
+  org had its documentation removed from every commit on 2026-08-08. SageTube is
+  a fork that still merges from `oop7/YTSage`, and a rewrite changes every commit
+  id — which would end that. The files leave in an ordinary commit instead.
+- `.github/` and `readme-translations/` are upstream's and were left alone, so
+  the next merge does not conflict over them.