Add CHANGELOG entry for Stylelint 15

CHANGELOG.md

@@ -8,6 +8,14 @@
 
 ## Unreleased
 
+This release upgrades to Stylelint 15 and now includes:
+
+* Checks for media query syntax, rules and values
+* Checks for number precision inside CSS functions
+* Fixes for duplicate rules with intentionally different units
+
+Read about breaking changes in the Stylelint [**Migrating to v15.0.0**](https://github.com/stylelint/stylelint/blob/main/docs/migration-guide/to-15.md#breaking-changes) guide.
+
 ### Breaking change: Removal of stylistic rules
 
 We have removed all stylistic rules (such as tabs/spaces, indentation, etc) as they're no longer included in Stylelint common configurations as part of their [deprecation in Stylelint 15]((https://stylelint.io/migration-guide/to-15/#deprecated-stylistic-rules)).