Update UPGRADE-6.0.md (#823)

Add missing breaking change to upgrade guide

Fixes #822

---------

Co-authored-by: Copilot <[email protected]>

Author: bbrala

CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Fix objects are non-unique despite key order ([#819](https://github.com/jsonrainbow/json-schema/pull/819))
 
+### Changed
+- Added extra breaking change to UPDATE-6.0.md regarding BaseConstraint::addError signature change ([#823](https://github.com/jsonrainbow/json-schema/pull/823)
+
 ## [6.4.1] - 2025-04-04
 ### Fixed
 - Fix support for 32bits PHP ([#817](https://github.com/jsonrainbow/json-schema/pull/817))

UPGRADE-6.0.md

@@ -24,4 +24,22 @@ Thank you for your support and contributions to the project.
     foreach ($validator->getErrors() as $error) {
         echo $error['constraint']['name']; // required
     }
-    ```
+    ```
+
+## BaseConstraint::addError signature changed
+
+* The signature for the `BaseConstraint::AddError` method has changed.
+
+  The `$message` parameter has been removed and replaced by the `ConstraintError` parameter. 
+  The `ConstraintError` object encapsulates the error message along with additional information about the constraint violation.
+
+    *Before*
+    ```php
+    public function addError(?JsonPointer $path, $message, $constraint = '', ?array $more = null)
+    ```
+
+    *After*
+    ```php
+    public function addError(ConstraintError $constraint, ?JsonPointer $path = null, array $more = []): void
+    ```
+