Refer to optional exercises from episode on code coverage

_episodes/22-scaling-up-unit-testing.md

@@ -350,4 +350,8 @@ Using automated testing where appropriate can save us considerable time,
 especially in the long term,
 and allows others to verify against correct behaviour.
 
+## Optional exercises
+Checkout [these optional exercises]({{ site.baseurl }}{% link _episodes/25-section2-optional-exercises.md %}#exercise-customize-pylint)
+to learn more about code coverage.
+
 {% include links.md %}