Update changelog

Author: thecrypticace

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Ensure validation of `source(…)` happens relative to the file it is in ([#19274](https://github.com/tailwindlabs/tailwindcss/pull/19274))
+
 ### Added
 
 - _Experimental_: Add `@container-size` utility ([#18901](https://github.com/tailwindlabs/tailwindcss/pull/18901))