Add null safety README to mockito.
This documents the code generation feature, and why its needed.
This is a tricky balance between getting the users up-and-running with code generation, and explaining why its needed, because some may consider it a pain, if its their first introduction to build_runner.
PiperOrigin-RevId: 316549387
diff --git a/README.md b/README.md
index dcb3565..c54bdca 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,11 @@
By declaring a class which extends Mockito's Mock class and implements the Cat
class under test, we have a class which supports stubbing and verifying.
+**Using Dart's new null safety feature?** Read the [NULL_SAFETY_README][] for
+help on creating mocks of classes featuring non-nullable types.
+
+[NULL_SAFETY_README]: https://github.com/dart-lang/mockito/blob/master/NULL_SAFETY_README.md
+
## Let's verify some behaviour!
```dart
