Update codebase-changes.md

Author: irenelagno

guides/v2.1/extension-dev-guide/versioning/codebase-changes.md

@@ -70,6 +70,7 @@ Use this table to understand what changes Magento can make and which version num
 | | Interface removed| MAJOR|
 | | Method removed | MAJOR|
 | | New required method argument | MAJOR|
+| | New optional method argument | MAJOR|
 | | Removed the last argument for a method | MINOR|
 | | Changed a method signature (excluding last argument removal) | MAJOR|
 | | New method exception (excluding subtypes of existing exceptions) | MAJOR|