Add changelog notes for v1.2.1

Author: the-mikedavis

CHANGELOG.md

@@ -6,6 +6,13 @@ The format is based on [Keep a
 Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 1.2.1 - 2025-12-08
+
+### Fixed
+
+- Fixed sample code in `c:Slipstream:init/1` ([#77](https://github.com/NFIBrokerage/slipstream/pull/77))
+- Fixed struct update compatibility with Elixir 1.19+ ([#82](https://github.com/NFIBrokerage/slipstream/pull/82))
+
 ## 1.2.0 - 2025-02-04
 
 ### Added