Adds note for pull requests handling.

Change-Id: I813a9b9de1f2bfbe72f41c30e612ef6530847444

Author: zhouyiself

README.md

@@ -1,3 +1,5 @@
 # MLKit Samples
 
 A collection of quickstart samples demonstrating the [ML Kit](https://developers.google.com/ml-kit) APIs on Android and iOS.
+
+Note: due to how this repo works, we no longer accept pull requests directly. Instead, we'll patch them internally and then sync them out.