Add constrained optimization entry to quickstart section.

Author: till-m

README.md

@@ -42,6 +42,8 @@ to learn how to make the package more flexible, how to deal with categorical par
 - Check out this
 [notebook](https://github.com/fmfn/BayesianOptimization/blob/master/examples/visualization.ipynb)
 with a step by step visualization of how this method works.
+- To understand how to use bayesian optimization when additional constraints are present, see the
+[constrained optimization notebook](https://github.com/fmfn/BayesianOptimization/blob/master/examples/constraints.ipynb).
 - Explore this [notebook](https://github.com/fmfn/BayesianOptimization/blob/master/examples/exploitation_vs_exploration.ipynb)
 exemplifying the balance between exploration and exploitation and how to
 control it.