Add documentation for Error Tracking

doc/user/project/operations/error_tracking.md

+# Error Tracking
+
+> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/169) in GitLab 11.7.
+
+Error tracking allows developers to easily discover and view the errors that their application may be generating. By surfacing error information where the code is being developed, efficiency and awareness can be increased.
+
+## Sentry error tracking
+
+[Sentry](https://sentry.io/) is an open source error tracking system. GitLab allows administrators to connect Sentry to GitLab, to allow users to view a list of Sentry errors in GitLab itself.
+
+### Deploying Sentry
+
+You may sign up to the cloud hosted https://sentry.io or deploy your own [on-premise instance](https://docs.sentry.io/server/installation/).
+
+### Enabling Sentry
+
+GitLab provides an easy way to connect Sentry to your project:
+
+1. Sign up to Sentry.io or [deploy your own](#deploying-sentry) Sentry instance.
+1. [Find or generate](https://docs.sentry.io/api/auth/) a Sentry auth token for your Sentry project.
+1. Navigate to your project’s **Settings > Operations** and provide the Sentry API URL and auth token.
+1. Ensure that the 'Active' checkbox is set.
+1. Click **Save changes** for the changes to take effect.
+1. You can now visit **Operations > Error Tracking** in your project's sidebar to [view a list](#error-tracking-list) of Sentry errors.
+
+## Error Tracking List
+
+The Error Tracking list may be found at **Operations > Error Tracking** in your project's sidebar.
+
+![Error Tracking list](img/error_tracking_list.png)

doc/user/project/settings/index.md

@@ -122,3 +122,9 @@ GitLab administrators can use the admin interface to move any project to any
 namespace if needed.
 
 [permissions]: ../../permissions.md##project-members-permissions
+
+## Operations settings
+
+### Error Tracking
+
+Configure Error Tracking to discover and view [Sentry errors within GitLab](../operations/error_tracking.md).