chore: update changelog.md

Author: cossssmin

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [5.1.0] - 2025-06-13
+
+This release adds support for using Tailwind CSS color opacity modifiers in Gmail by automatically converting modern rgb/a syntax to the legacy, comma-separated syntax.
+
+- feat: use commas instead of slash for css color opacity modifiers f26774b
+
 ## [5.0.9] - 2025-06-12
 
 This release fixes an issue where builds failed because `preferUnitlessValues` (which is on by default) was throwing an error when encountering invalid inline CSS.