Add unique to the README example

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

README.md

@@ -98,11 +98,13 @@ Define enums with deprecated members that raise deprecation warnings when
 accessed:
 
 ```python
-from frequenz.core.enum import Enum, deprecated_member
+from frequenz.core.enum import Enum, deprecated_member, unique
 
+@unique
 class TaskStatus(Enum):
    OPEN = 1
    IN_PROGRESS = 2
+   # Duplicate values are fine with `@unique` as long as they are deprecated
    PENDING = deprecated_member(1, "PENDING is deprecated, use OPEN instead")
    DONE = deprecated_member(3, "DONE is deprecated, use FINISHED instead")
    FINISHED = 4