Adds a readme file for R1 folder.

PiperOrigin-RevId: 290663511

official/r1/README.md

+# Legacy Models Collection
+
+The R1 folder contains legacy model implmentation and models that will not
+update to TensorFlow 2.x. They do not have solid performance tracking.
+
+## Legacy model implmentation
+
+Transformer and MNIST implementation uses pure TF 1.x TF-Estimator.
+Users should follow the corresponding TF 2.x implmentation inside the
+official model garden.
+
+## Models that will not update to TensorFlow 2.x
+
+*   [boosted_trees](boosted_trees): A Gradient Boosted Trees model to
+    classify higgs boson process from HIGGS Data Set.
+*   [wide_deep](wide_deep): A model that combines a wide model and deep
+    network to classify census income data.