Merge branch 'docs-custom-issue-tracker' into 'master'

Custom issue tracker

See merge request gitlab-org/gitlab-ce!15516

doc/integration/external-issue-tracker.md

@@ -22,6 +22,7 @@ Visit the links below for details:
 - [Redmine](../user/project/integrations/redmine.md)
 - [Jira](../user/project/integrations/jira.md)
 - [Bugzilla](../user/project/integrations/bugzilla.md)
+- [Custom Issue Tracker](../user/project/integrations/custom_issue_tracker.md)
 
 ### Service Template
 

doc/user/project/integrations/custom_issue_tracker.md

+# Custom Issue Tracker Service
+
+To enable the Custom Issue Tracker integration in a project, navigate to the
+[Integrations page](project_services.md#accessing-the-project-services), click
+the **Customer Issue Tracker** service, and fill in the required details on the page as described
+in the table below.
+
+| Field | Description |
+| ----- | ----------- |
+| `title`   | A title for the issue tracker (to differentiate between instances, for example) |
+| `description`   | A name for the issue tracker (to differentiate between instances, for example) |
+| `project_url`   | Currently unused. Will be changed in a future release. |
+| `issues_url`    | The URL to the issue in the issue tracker project that is linked to this GitLab project. Note that the `issues_url` requires `:id` in the URL. This ID is used by GitLab as a placeholder to replace the issue number. For example, `https://customissuetracker.com/project-name/:id`. |
+| `new_issue_url` | Currently unused. Will be changed in a future release. |
+
+
+## Referencing issues
+
+Issues are referenced with `#<ID>`, where `<ID>` is a number (example `#143`). 
+So with the example above, `#143` would refer to `https://customissuetracker.com/project-name/143`.
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