Update policies doc to require constructor stubbing

POLICIES.md

 
 ### Starter implementations
 
-> - Exercises 1-10: provide stubs for all required methods. Stubs should include the following body:
+> - Exercises 1-10: 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 11-20: provide stubs for all methods required by the first test; comment-out the bodies of any tests that require non-stubbed methods. Stubs should include the following body:
+> - Exercises 11-20: provide stubs for all constructors and methods required by the first test; comment-out the bodies of any tests that require non-stubbed constructors or methods. Stubs should include the following body:
     `throw new UnsupportedOperationException("Delete this statement and provide your own implementation.");`
 > - Exercises 21+: provide no stubs, but mention any "interesting" interface aspects in the HINT.md file (which gets merged into the README.md for the exercise).