Update CHANGELOG

Author: dhardy

CHANGELOG.md

@@ -4,6 +4,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+- New `pub type RawOsError` [#739]
+
+[#739]: https://github.com/rust-random/getrandom/pull/739
+
 ## [0.3.4] - 2025-10-14
 
 ### Major change to `wasm_js` backend