Add Issue Type documentation

* basic draft about usage of issue types

doc/development/README.md

@@ -71,6 +71,7 @@ description: 'Learn how to contribute to GitLab.'
 - [Auto DevOps development guide](auto_devops.md)
 - [Mass Inserting Models](mass_insert.md)
 - [Cycle Analytics development guide](cycle_analytics.md)
+- [Issue types vs first-class types](issue_types.md)
 
 ## Performance guides
 

doc/development/issue_types.md

+# Issue Types
+
+Sometimes when a new resource type is added it's not clear if it should be only an
+"extension" of Issue (Issue Type) or if it should be a new first-class resource type
+(similar to Issue, Epic, Merge Request, Snippet).
+
+The idea of Issue Types was first proposed in [this
+issue](https://gitlab.com/gitlab-org/gitlab/issues/8767) and its usage was
+discussed few times since then, for example in [incident
+management](https://gitlab.com/gitlab-org/gitlab-foss/issues/55532).
+
+## What is an Issue Type
+
+Issue Type is a resource type which extends the existing Issue type and can be
+used anywhere where Issue is used - for example when listing or searching
+issues or when linking objects of the type from Epics. It should use the same
+`issues` table, additional fields can be stored in a separate table.
+
+## When an Issue Type should be used
+
+- When the new type only adds new fields to the basic Issue type without
+  removing existing fields (but it's OK if some fields from the basic Issue
+  type are hidden in user interface/API).
+- When the new type can be used anywhere where the basic Issue type is used.
+
+## When a first-class resource type should be used
+
+- When a separate model and table is used for the new resource.
+- When some fields of the basic Issue type need to be removed - hiding in the UI
+  is OK, but not complete removal.
+- When the new resource cannot be used instead of the basic Issue type,
+  for example:
+
+  - You can't add it to an epic.
+  - You can't close it from a commit or a merge request.
+  - You can't mark it as related to another issue.
+
+If an Issue type can not be used you can still define a first-class type and
+then include concerns such as `Issuable` or `Noteable` to reuse functionality
+which is common for all our issue-related resources. But you still need to
+define the interface for working with the new resource and update some other
+components to make them work with the new type.
+
+Usage of the Issue type limits what fields, functionality, or both is available
+for the type. However, this functionality is provided by default.