Documentation fix for issue #2307

This is the proposed addition to the doc to describe the need to have a title field, or set up an alias for models in custom Refinery engines. The copy is as follows:

TIP: Models in Refinery engines expect a string field that acts as the title identifier when displayed in lists in the admin pages. If a title field is not included, the first string field found will be used. Models without a usable field for a title will cause the admin to raise an error, so please include a title field or alias another field to "title" using the alias_attribute method when creating models in your engine.

Sorry I'm late on this, and thanks for Refinery