POLICIES: update stub policy for nextercism

POLICIES.md

 
 ### Starter implementations
 
-> - Exercises 1-20: provide stubs for all required constructors and methods. Stubs should include the following body:
+> - Exercises of difficulty 4 or lower: provide stubs for all required constructors and methods. Stubs should include the following body:
     `throw new UnsupportedOperationException("Delete this statement and write your own implementation.");`
-> - Exercises 21+: provide no stubs by default, but either (1) add hints to the hints.md file (which gets merged into the README.md for the exercise) or (2) provide stubs as in exercises 1-20 for exercises that demand complicated method signatures.
+> - Exercises of difficulty 5 or higher: provide no stubs by default, but either (1) add hints to the hints.md file (which gets merged into the README.md for the exercise) or (2) provide stubs as above for exercises that demand complicated method signatures.
 
-References: [[1](https://github.com/exercism/java/issues/178)], [[2](https://github.com/exercism/java/pull/683#discussion_r125506930)]
+References: [[1](https://github.com/exercism/java/issues/178)], [[2](https://github.com/exercism/java/pull/683#discussion_r125506930)], [[3](https://github.com/exercism/java/issues/977)]
 
 ### Avoid using final