Update key scoring warning to specify future default behaviour change

mir_eval/key.py

@@ -146,7 +146,8 @@ def weighted_score(reference_key, estimated_key,
     if not allow_descending_fifths:
         warnings.warn('The selected key scoring method does not match that '\
             'currently used by MIREX. To use the same method, specify '\
-            'allow_descending_fifths=True.')
+            'allow_descending_fifths=True. The default behaviour will '\
+            'change to allow_descending_fifths=True in the future.')
 
     validate(reference_key, estimated_key)
     reference_key, reference_mode = split_key_string(reference_key)