Resurrect a useful comment

ARISE NOW, YE TARNISHED

Author: David Robertson

signedjson/types.py

@@ -59,5 +59,7 @@ def sign(self, message):
 
     @property
     def verify_key(self):
+        # Note: use `signedjson.key.get_verify_key` to get a
+        # `signedjson.types.VerifyKey`.
         # type: () -> nacl.signing.VerifyKey
         pass  # pragma: nocover