message: Link to Datasets doc in release notes
diff: # Release 1.4.0
  ## Major Features And Improvements
  -* `tf.data` is now part of the core TensorFlow API.
  +* [`tf.data`](http://tensorflow.org/programmers_guide/datasets) is now part of
  + the core TensorFlow API.
  * The API is now subject to backwards compatibility guarantees.
  * For a guide to migrating from the `tf.contrib.data` API, see the
  [README] (https://github.com/tensorflow/tensorflow/blob/r1.4/tensorflow/contrib/data/README.md).
  