Note about Flutter version

README.md

@@ -7,6 +7,10 @@ request or found a bug, please file it at the [issue tracker][].
 
 [issue tracker]: https://github.com/memspace/zefyr/issues
 
+> **Important:** Zefyr currently tracks latest development version of
+> Flutter (`master` branch) and may not work with Flutter beta channel
+> if there are breaking changes.
+
 ### Documentation
 
 * [Quick Start][quick_start]