Add CHANGELOG and document the update process

.gitattributes

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+CHANGELOG.md merge=union

CHANGELOG.md

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+<!--
+High-level release notes.
+Loosely based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+When your PR includes a user-facing change, add an entry below under the
+appropriate heading (create the heading if it does not yet exist). Within
+each heading content can be free-form. Feel free to include examples, links
+to docs, or any other relevant information.
+
+### Added            — new features
+### Changed          — changes in existing functionality
+### Deprecated       — soon-to-be-removed features
+### Breaking Changes — removed or backwards-incompatible features
+### Fixed            — notable bug fixes
+### Security         — notable security fixes
+-->
+
+# Changelog
+
+Release notes for versions released before this file was introduced are archived
+under [`releases/`](releases/).
+
+## [Unreleased]

CONTRIBUTING.md

@@ -80,3 +80,17 @@ softwareupdate --install-rosetta
 ```
 
 for builds to complete successfully.
+
+## Changelog
+
+User-facing changes are recorded in [`CHANGELOG.md`](CHANGELOG.md), loosely following the
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/) format. (Release notes for older versions
+are archived under [`releases/`](releases/).)
+
+If your PR includes a user-facing change (new feature, behavior change, deprecation, breaking
+change, notable bug fix, or security fix), add a short, high-level entry to the `## [Unreleased]`
+section at the top of `CHANGELOG.md` under the appropriate heading, creating it if needed:
+Added, Changed, Deprecated, Breaking Changes, Fixed, or Security.
+
+Keep entries high-level and written for users. The full commit log is appended at release time,
+so internal-only changes (refactors, tests, CI, docs) don't need an entry.