Add example of subclass redefining a parent key to README.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dblock

README.md

@@ -270,7 +270,17 @@ ShippedOrderState.key?(:CREATED) # true
 ShippedOrderState.value(:CREATED) # 'CREATED'
 ```
 
-A sub class may redefine a key or value already used by a parent class; its own definition takes precedence.
+A sub class may redefine a key or value already used by a parent class without raising `DuplicateKeyError` or `DuplicateValueError`; its own definition takes precedence.
+
+``` ruby
+class ShippedOrderState < OrderState
+  define :CREATED, 'RECREATED' # does not raise, overrides the parent class' definition
+end
+
+ShippedOrderState::CREATED # 'RECREATED'
+ShippedOrderState.value(:CREATED) # 'RECREATED'
+OrderState.value(:CREATED) # 'CREATED', unaffected
+```
 
 ### Exhaustive case matcher