Add a note to the official README that users should upgrade to TF1.4 (tensorflow#2758)

Author: nealwu

official/README.md

@@ -2,7 +2,11 @@
 
 The TensorFlow official models are a collection of example models that use TensorFlow's high-level APIs. They are intended to be well-maintained, tested, and kept up to date with the latest stable TensorFlow API. They should also be reasonably optimized for fast performance while still being easy to read.
 
-Below is the list of models contained in the garden:
+Currently the models are compatible with TensorFlow 1.4. If you are on an earlier version please [update your installation](https://www.tensorflow.org/install/).
+
+---
+
+Below is a list of the models available.
 
 [mnist](mnist): A basic model to classify digits from the MNIST dataset.