MAINT add link to example

Author: KEggensperger

doc/manual.rst

@@ -21,6 +21,8 @@ aspects of its usage:
 * `Sequential usage <https://github.com/automl/auto-sklearn/blob/master/example/example_sequential.py>`_
 * `Regression <https://github.com/automl/auto-sklearn/blob/master/example/example_regression.py>`_
 * `Continuous and Categorical Data <https://github.com/automl/auto-sklearn/blob/master/example/example_feature_types.py>`_
+* `Using Custom metrics <https://github.com/automl/auto-sklearn/blob/master/example/example_metrics.py>`_
+
 
 Time and memory limits
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