Update QA docs re. per-test feature flags

Document that individual tests can be run with a feature flag toggled

doc/development/testing_guide/end_to_end/feature_flags.md

+# Testing with feature flags
+
+To run a specific test with a feature flag enabled you can use the `QA::Runtime::Feature` class to enabled and disable feature flags ([via the API](../../../api/features.md)).
+
+```ruby
+context "with feature flag enabled" do
+  before do
+    Runtime::Feature.enable('feature_flag_name')
+  end
+
+  it "feature flag test" do
+    # Execute a test with a feature flag enabled
+  end
+
+  after do
+    Runtime::Feature.disable('feature_flag_name')
+  end
+end
+```
+
+## Running a scenario with a feature flag enabled
+
+It's also possible to run an entire scenario with a feature flag enabled, without having to edit existing tests or write new ones.
+
+Please see the [QA readme](https://gitlab.com/gitlab-org/gitlab/tree/master/qa#running-tests-with-a-feature-flag-enabled) for details.

doc/development/testing_guide/end_to_end/index.md

@@ -130,6 +130,7 @@ Continued reading:
 - [Quick Start Guide](quick_start_guide.md)
 - [Style Guide](style_guide.md)
 - [Best Practices](best_practices.md)
+- [Testing with feature flags](feature_flags.md)
 
 ## Where can I ask for help?