add some documentation for `triggers` keyword [ci skip]

doc/ci/yaml/README.md

@@ -286,7 +286,7 @@ There are a few rules that apply to the usage of refs policy:
 * `only` and `except` are inclusive. If both `only` and `except` are defined
    in a job specification, the ref is filtered by `only` and `except`.
 * `only` and `except` allow the use of regular expressions.
-* `only` and `except` allow the use of special keywords: `branches` and `tags`.
+* `only` and `except` allow the use of special keywords: `branches`, `tags`, and `triggers`.
 * `only` and `except` allow to specify a repository path to filter jobs for
    forks.
@@ -303,6 +303,17 @@ job:
     - branches
 ```
+In this example, `job` will run only for refs that are tagged, or if a build is explicitly requested
+via an API trigger.
+
+```yaml
+job:
+  # use special keywords
+  only:
+    - tags
+    - triggers
+```
+
 The repository path can be used to have jobs executed only for the parent
 repository and not forks: