Add changelog file.

Author: Crell

CHANGELOG.md

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+# Changelog
+
+All notable changes to `ApiProblem` will be documented in this file.
+
+Updates should follow the [Keep a CHANGELOG](http://keepachangelog.com/) principles.
+
+## 3.7.1 - 2026-01-12
+
+### Added
+- Changed PHPDocs to use `$this` for fluent methods, which helps IDE autocompletes.
+
+### Deprecated
+- Nothing
+
+### Fixed
+- Assorted legalese and paperwork updates.
+- Improved .gitattributes to ship fewer files.
+
+### Removed
+- Nothing
+
+### Security
+- Nothing