message: Add release note: sparse tensors in `tf.data`
diff: * Make `tf.contrib.distributions` QuadratureCompound classes support batch.
  * `Stream::BlockHostUntilDone` now returns Status rather than bool.
  * Customize request timeouts for the GCS filesystem.
  +* `tf.data` now supports `tf.SparseTensor` components in dataset elements.
  ## Thanks to our Contributors
  