Link to end to end test guideline from test pyramid

doc/development/testing_guide/testing_levels.md

@@ -121,6 +121,9 @@ running feature tests (i.e. using Capybara) against it.
 The actual test scenarios and steps are [part of GitLab Rails] so that they're
 always in-sync with the codebase.
 
+Read a separate document about [end-to-end tests](../end_to_end_tests.md) to
+learn more.
+
 [multiple pieces]: ../architecture.md#components
 [GitLab Shell]: https://gitlab.com/gitlab-org/gitlab-shell
 [GitLab Workhorse]: https://gitlab.com/gitlab-org/gitlab-workhorse