Adding note about test credit cards in PP Pro sandbox.

paypal/pro/creditcard.py

@@ -50,6 +50,9 @@ def is_mod10(self):
 
     def is_test(self):
         """Returns True if number is a test card number."""
+        # Note: test numbers cannot be used in the PP Pro sandbox.
+        # Instead, use the credit card number associated with a
+        # sandbox account (Test Accounts -> View Details).
         return self.number in TEST_NUMBERS
 
     def get_type(self):
@@ -63,4 +66,4 @@ def verify(self):
         """Returns the card type if valid else None."""
         if self.is_number() and not self.is_test() and self.is_mod10():
             return self.get_type()
-        return None
+        return None