message: Add a note in docs about 2.0 alpha differing for TFLiteConverter.
diff: @@ -18,6 +18,12 @@ classmethods to convert a model based on the original model format:
  * `TFLiteConverter.from_concrete_functions()`: Converts
  [concrete functions](concrete_function.md).
  +Note: The TensorFlow Lite 2.0 alpha had a different version of the
  +`TFLiteConverter` API which only contained the classmethod
  +[`from_concrete_function`](https://www.tensorflow.org/versions/r2.0/api_docs/python/tf/lite/TFLiteConverter#from_concrete_function).
  +The API detailed in this document can be installed using the
  +[`tf-nightly-2.0-preview`](#installing_the_tensorflow_20_nightly_) pip install.
  +
  This document contains [example usages](#examples) of the API, a detailed list
  of [changes in the API between 1.X and 2.0](#differences), and
  [instructions](#versioning) on running the different versions of TensorFlow.
  