Add custom_field module to supported relationships

Hi, i'm the maintainer for the Custom Field → module. As of latest released version, I have added a plugin that integrates with Entity Usage module to track referenced entities. I though you might consider noting it for your users on the project page if you are interested.

Proposed bullet to add:

Entities related through <a href="https://www.drupal.org/project/custom_field" title="Custom Field">Custom Field</a> entity_reference sub-fields