chore: clarify difficulty field.

README.md

@@ -23,7 +23,7 @@ Large Language Models (LLMs) have shown impressive performance on various benchm
 - `answerDescription` - the group name
 - `words` - the 4 words that classify into this group
 
-`difficulty` - the difficulty of the puzzle as rated by community contributors.
+`difficulty` - the difficulty of the puzzle as rated by community contributors (should such a rating be obtained, otherwise `null`).
 
 ## Citation
 ```