Add section of enabling GitLab CI

doc/ci/quick_start/README.md

@@ -21,6 +21,13 @@ page.
 Now, let's break it down to pieces and work on solving the GitLab CI puzzle.
+## 1. Enable GitLab CI
+
+After creating a new project, the first thing to do is enable the **GitLab CI**
+service in your project's settings if it isn't already enabled.
+
+Read [how to enable the GitLab CI service](enable_ci.md).
+
 ## 1. Creating a `.gitlab-ci.yml` file
  **GitLab CI** service is enabled automatically on the first push of a

doc/ci/quick_start/enable_ci.md

+# Enable GitLab CI
+
+GitLab Continuous Integration (CI) is fully integrated into GitLab itself. You
+only need to enable it in the **Services** settings of your project.
+
+First, head over your project's page that you would like to enable CI for.
+If you can see the **Builds** tab in the sidebar, then CI is enabled.
+
+![Builds tab](builds_tab.png)
+
+If not, go to **Settings > Services** and search for **GitLab CI**. Its state
+should be disabled.
+
+![CI service disabled](ci_service_disabled.png)
+
+Click on **GitLab CI** to enter its settings, mark it as active and hit
+**Save**.
+
+![Mark CI service as active](ci_service_mark_active.png)
+
+Do you see that green dot? Then good, the service is now enabled! You can also
+check its status under **Services**.
+
+![CI service enabled](ci_service_enabled.png)