add changelog

Author: cenkalti

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/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [5.0.0] - 2024-12-19
+
+### Added
+
+- RetryAfterError can be returned from an operation to indicate how long to wait before the next retry.
+
+### Changed
+
+- Retry function now accepts additional options for specifying max number of tries and max elapsed time.
+- Retry function now accepts a context.Context.
+- Operation function signature changed to return result (any type) and error.
+
+### Removed
+
+- RetryNotify* and RetryWithData functions. Only single Retry function remains.
+- Optional arguments from ExponentialBackoff constructor.
+- Clock and Timer interfaces.
+
+### Fixed
+
+- The original error is returned from Retry if there's a PermanentError. (#144)
+- The Retry function respects the wrapped PermanentError. (#140)