Added orderBy to child mapping in .yml entity mapping example. Omitting this causes children to be pulled in the wrong order which can be confusing.

doc/tree.md

@@ -263,6 +263,8 @@ Entity\Category:
     children:
       targetEntity: Entity\Category
       mappedBy: parent
+      orderBy:
+        lft: ASC
 ```
 
 <a name="xml-mapping"></a>