Add doc for scheduled trigger

app/views/projects/triggers/_form.html.haml

@@ -19,7 +19,7 @@
               %strong Schedule trigger (experimental)
             .help-block
               If checked, this trigger will be executed periodically according to cron and timezone.
-              = link_to icon('question-circle'), help_page_path('ci/triggers', anchor: 'schedule')
+              = link_to icon('question-circle'), help_page_path('ci/triggers/README', anchor: 'using-scheduled-triggers')
         .form-group
           = schedule_fields.label :cron, "Cron", class: "label-light"
           = schedule_fields.text_field :cron, class: "form-control", title: 'Cron specification is required.', placeholder: "0 1 * * *"

doc/ci/triggers/README.md

@@ -227,3 +227,31 @@ branch of project with ID `9` every night at `00:30`:
 ```
 [ci-229]: https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/229
+
+## Using scheduled triggers
+
+> [Introduced][ci-10533] in GitLab CE 9.1 as experimental.
+
+In order to schedule a trigger, navigate to your project's **Settings ➔ CI/CD Pipelines ➔ Triggers** and edit an existing trigger token.
+
+![Triggers Schedule edit](img/trigger_schedule_edit.png)
+
+To set up a scheduled trigger:
+
+1. Check the **Schedule trigger (experimental)** checkbox
+1. Enter a cron value for the frequency of the trigger ([learn more about cron notation](http://www.nncron.ru/help/EN/working/cron-format.htm))
+1. Enter the timezone of the cron trigger ([see a list of timezones](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones))
+1. Enter the branch or tag that the trigger will target
+1. Hit **Save trigger** for the changes to take effect
+
+![Triggers Schedule create](img/trigger_schedule_create.png)
+
+You can check a next execution date of the scheduled trigger, which is automatically calculated by a server.
+
+![Triggers Schedule create](img/trigger_schedule_updated_next_run_at.png)
+
+> **Notes**:
+- Those triggers won't be executed precicely. Because scheduled triggers are handled by Sidekiq, which runs according to its interval. For exmaple, if you set a trigger to be executed every minute (`* * * * *`) and the Sidekiq worker performs 00:00 and 12:00 o'clock every day (`0 */12 * * *`), then your trigger will be executed only 00:00 and 12:00 o'clock every day. To change the Sidekiq worker's frequency, you have to edit the `trigger_schedule_worker` value in `config/gitlab.yml` and restart GitLab. The Sidekiq worker's configuration on GiLab.com is able to be looked up at [here](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/gitlab.yml.example#L185).
+- Cron notation is parsed by [Rufus-Scheduler](https://github.com/jmettraux/rufus-scheduler).
+
+[ci-10533]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10533