Add a note for the plan to delete R1 folder.

PiperOrigin-RevId: 300590317

official/r1/README.md

@@ -3,6 +3,12 @@
 The R1 folder contains legacy model implmentation and models that will not
 update to TensorFlow 2.x. They do not have solid performance tracking.
 
+**Note: models will be removed from the master branch by 2020/06.**
+
+After removal, you can still access to these legacy models in the previous
+released tags, e.g. [v2.1.0](https://github.com/tensorflow/models/releases/tag/v2.1.0).
+
+
 ## Legacy model implmentation
 
 Transformer and MNIST implementation uses pure TF 1.x TF-Estimator.