Document triggers in yaml/README.md [ci skip]

doc/ci/yaml/README.md

@@ -66,6 +66,7 @@ There are a few reserved `keywords` that **cannot** be used as job names:
 | before_script | no | Define commands that run before each job's script |
 | variables     | no | Define build variables |
 | cache         | no | Define list of files that should be cached between subsequent runs |
+| trigger       | no | Force a rebuild of a specific branch or tag with an API call |
 
 ### image and services
 
@@ -152,6 +153,32 @@ cache:
   - binaries/
 ```
 
+### trigger
+
+Triggers can be used to force a rebuild of a specific branch or tag with an API
+call. You can add a trigger by visiting the project's **Settings > Triggers**.
+
+Every new trigger you create, gets assigned a different token which you can
+then use inside your `.gitlab-ci.yml`:
+
+```yaml
+trigger:
+  stage: deploy
+  script:
+    - "curl -X POST -F token=TOKEN -F ref=master https://gitlab.com/api/v3/projects/9/trigger/builds"
+```
+
+In the example above, a rebuild on master branch will be triggered after all
+previous stages build successfully (denoted by `stage: deploy`).
+
+You can find the endpoint containing the ID of the project on the **Triggers**
+page.
+
+_**Warning:** Be careful how you set up your triggers, because you could end up
+in an infinite loop._
+
+Read more in the dedicated [triggers documentation](../triggers/README.md).
+
 ## Jobs
 
 `.gitlab-ci.yml` allows you to specify an unlimited number of jobs. Each job