docs: UPGRADE.md

Author: disstudio

UPGRADE.md

@@ -1,5 +1,31 @@
 # UPGRADE
 
+## From v0.2.0 to v0.2.1
+
+### Configuration key serialization_group renamed to serialization_groups
+
+The configuration key for export resources has been renamed to reflect that it can handle multiple groups.
+
+Before:
+
+```yaml
+sylius_import_export:
+    export:
+        resources:
+            app.brand:
+                serialization_group: 'app:brand:export'
+```
+
+After:
+
+```yaml
+sylius_import_export:
+    export:
+        resources:
+            app.brand:
+                serialization_groups: ['app:brand:export']
+```
+
 ## From v0.1.x to v0.2.0
 
 ### `BatchedExportDataManager` constructor signature changed