Reference Issue #581 in relevant section of README

README.md

@@ -346,7 +346,8 @@ with `when(cat.sound())`, so what should the code do? What is the "missing stub"
 behavior?
 
 * The "missing stub" behavior of a mock class generated with `@GenerateMocks` is
-  to throw an exception.
+  to throw an exception (with the exception of methods that return
+  `Future<void>` – see https://github.com/dart-lang/mockito/issues/581).
 * The "missing stub" behavior of a mock class generated with
   `@GenerateNiceMocks` is to return a "simple" legal value (for example, a
   non-`null` value for a non-nullable return type). The value should not be used