add null safety guide to readme (#299)
diff --git a/packages/pigeon/README.md b/packages/pigeon/README.md
index 30fa261..699cffd 100644
--- a/packages/pigeon/README.md
+++ b/packages/pigeon/README.md
@@ -110,6 +110,12 @@
}
```
+## Null Safety (NNBD)
+
+In order to generate null-safe code run the command line with the extra argument
+`--dart_null_safety`. For example:
+`flutter pub run pigeon --input ./pigeons/messages.dart --dart_null_safety`.
+
## Feedback
File an issue in [flutter/flutter](https://github.com/flutter/flutter) with the
