Add developer documentation about Issue Types vs first-class Issuable resources
Sometimes when a new resource 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 which re-uses existing code we have in concerns used by Issue ad other Issuable resources.
It would be best to add developer documentation with basic rules for deciding when to use which type. It would be nice to also document which parts of code should be usually updated when a new "Issue-like" resource is added.
Good starting point might be https://gitlab.com/gitlab-org/gitlab-ce/issues/55532#note_126495169, and also related slack thread: https://gitlab.slack.com/archives/C72HPNV97/p1567614997065000