[docs] Add backlinks from hyperopt / rl algorithm examples to the built-on Ray libraries (#1356)

doc/source/example-hyperopt.rst

@@ -2,6 +2,11 @@ Hyperparameter Optimization
 ===========================
 
 This document provides a walkthrough of the hyperparameter optimization example.
+
+.. note::
+
+     To learn about Ray's built-in hyperparameter optimization framework, see `Ray.tune <http://ray.readthedocs.io/en/latest/tune.html>`__.
+
 To run the application, first install some dependencies.
 
 .. code-block:: bash