Add instructions on how to add a new issue type

Add detailed instructions on how a user/developer can add a new issue_type. Some points to consider:
- Restrictions on where the new class is defined and when it is registered
- Clarify the user's self-defined IssueManager should have attribute issue_name set to the same key string that they specify here (otherwise some users may be confused that "Custom" is a special built-in key with some special properties rather than self-defined)