Add a note about feature flag `incident_management`

This section has been borrowed from https://gitlab.com/gitlab-org/gitlab-ee/blob/e9ea40bc82146e8102ae5e40ac40672ffa599c16/doc/user/project/merge_requests/merge_request_approvals.md

doc/user/project/integrations/prometheus.md

@@ -131,9 +131,6 @@ will appear, with options to set the threshold and operator. Click **Add** to sa
 
 ![Adding an alert](img/prometheus_alert.png)
 
-If the metric exceeds the threshold of the alert for over 5 minutes, an email
-will be sent to all [Maintainers and Owners](../../permissions.md#project-members-permissions) of the project.
-
 To remove the alert, click back on the alert icon for the desired metric, and click **Delete**.
 
 #### External Prometheus instances
@@ -157,6 +154,41 @@ receivers:
   ...
 ```
 
+### Taking action on an alert **[ULTIMATE]**
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/issues/4925) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 11.9.
+
+Alerts can be used to trigger actions, like open an issue automatically. To configure the actions:
+
+1. Navigate to your project's **Settings > Operations > Incidents**.
+1. Enable the option to create issues.
+1. Choose the [issue template](../description_templates.md) to create the issue from.
+1. Optionally, select whether to send an email notification to the developers of the project.
+1. Click **Save changes**.
+
+Once enabled, an issue will be opened automatically when an alert is triggered. To further customize the issue, you can add labels, mentions, or any other supported [quick action](../quick_actions.md) in the selected issue template.
+
+If the metric exceeds the threshold of the alert for over 5 minutes, an email will be sent to all [Maintainers and Owners](../../permissions.md#project-members-permissions) of the project.
+
+NOTE: **Note:**
+If you are running a self-managed instance, the new interface shown on
+this page will not be available unless the feature flag
+`incident_management` is enabled, which can be done from the Rails console by
+instance administrators.
+
+Use these commands to start the Rails console:
+
+```sh
+# Omnibus GitLab
+gitlab-rails console
+
+# Installation from source
+cd /home/git/gitlab
+sudo -u git -H bin/rails console RAILS_ENV=production
+```
+
+Then run `Feature.enable(:incident_management)` to enable the feature flag.
+
 ## Determining the performance impact of a merge
 
 > [Introduced][ce-10408] in GitLab 9.2.