Added note about new methods in UPGRADE.md

UPGRADE.md

@@ -33,6 +33,12 @@ Also, related functions were affected:
 Internal changes were made to DQL and SQL generation. If you have implemented your own TreeWalker,
 you probably need to update it. The method walkJoinVariableDeclaration is now named walkJoin.
 
+## New methods in TreeWalker interface *BC break*
+
+Two methods getQueryComponents() and setQueryComponent() were added to the TreeWalker interface and all its implementations
+including TreeWalkerAdapter, TreeWalkerChain and SqlWalker. If you have your own implementation not inheriting from one of the
+above you must implement these new methods.
+
 ## Metadata Drivers
 
 Metadata drivers have been rewritten to reuse code from Doctrine\Common. Anyone who is using the