Add docs about end-to-end testing / GitLab QA tests

doc/development/testing_guide/end_to_end_tests.md

+# End-to-End Testing
+
+## What is End-to-End testing?
+
+End-to-End testing is a strategy used to check whether your application works
+as expected across entire software stack and architecture, including
+integration of all microservices and components that are supposed to work
+together.
+
+## How do we test GitLab?
+
+We use [Omnibus GitLab][omnibus-gitlab] to build GitLab packages and then we
+test these packages using [GitLab QA][gitlab-qa] project, which is entirely
+black-box, click-driven testing framework.
+
+### Testing nightly builds
+
+We run scheduled pipeline each night to test nightly builds created by Omnibus.
+You can find these nightly pipelines at [GitLab QA pipelines page][gitlab-qa-pipelines].
+
+### Testing code in merge requests
+
+It is also possible to trigger packages build and [GitLab QA pipeline][gitlab-qa-pipelines]
+using a manual action that should be present in the merge request widget on
+your merge request. Look for `package-qa` manual action.
+
+Below you can read more about how to use it and how does it work.
+
+## How does it work?
+
+We are using _multi-project pipelines_ to run end-to-end tests.
+
+## How do I test my code?
+
+## How do I contribute?
+
+## Where can I ask for help?
+
+
+[omnibus-gitlab]: https://gitlab.com/gitlab-org/omnibus-gitlab
+[gitlab-qa]: https://gitlab.com/gitlab-org/gitlab-qa
+[gitlab-qa-pipelines]: https://gitlab.com/gitlab-org/gitlab-qa/pipelines

doc/development/testing_guide/index.md

@@ -65,6 +65,13 @@ Everything you should know about how to test Rake tasks.
 
 ---
 
+## [End-to-end tests](end_to_end_tests.md)
+
+Everything you should know about how to run end-to-end tests, also known as
+[GitLab QA][gitlab-qa] tests.
+
+---
+
 ## Spinach (feature) tests
 
 GitLab [moved from Cucumber to Spinach](https://github.com/gitlabhq/gitlabhq/pull/1426)
@@ -89,3 +96,4 @@ test should be re-implemented using RSpec instead.
 [Capybara]: https://github.com/teamcapybara/capybara
 [Karma]: http://karma-runner.github.io/
 [Jasmine]: https://jasmine.github.io/
+[gitlab-qa]: https://gitlab.com/gitlab-org/gitlab-qa