Add option to enable GitLab CI via .gitlab-ci.yml

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.
+GitLab Continuous Integration (CI) is fully integrated into GitLab itself as
+of [version 8.0](https://about.gitlab.com/2015/09/22/gitlab-8-0-released/).
 
-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.
+First, head over your project's page that you would like to enable GitLab CI
+for. If you can see the **Builds** tab in the sidebar, then GitLab CI is
+already enabled and you can skip reading the rest of this guide.
 
 ![Builds tab](builds_tab.png)
 
-If not, go to **Settings > Services** and search for **GitLab CI**. Its state
-should be disabled.
+If not, there are two ways to enable it in your project.
+
+## Use .gitlab-ci.yml to enable GitLab CI
+
+GitLab CI will be automatically enabled if you just push a
+[`.gitlab-ci.yml`](../yaml/README.md) in your git repository. GitLab will
+pick up the change immediately and GitLab CI will be enabled for this project.
+This is the recommended way.
+
+## Manually enable GitLab CI
+
+The second way is to manually enable it in the project's **Services** settings
+and this is also the way to disable it if needed.
+
+Go to **Settings > Services** and search for **GitLab CI**. Its state should
+be disabled.
 
 ![CI service disabled](ci_service_disabled.png)