Extend README.md for GitLab QA in `qa/` directory

qa/README.md

@@ -3,3 +3,16 @@
 This directory contains integration tests for GitLab.
 
 It is part of [GitLab QA project](https://gitlab.com/gitlab-org/gitlab-qa).
+
+## What GitLab QA is?
+
+GitLab QA is an integration tests suite for GitLab.
+
+These are black-box and entirely click-driven integration tests you can run
+against any existing instance.
+
+## How does it work?
+
+1. When we release a new version of GitLab, we build a Docker images for it.
+1. Along with GitLab Docker Images we also build and publish GitLab QA images.
+1. GitLab QA project uses these images to execute integration tests.