Document role assignment and other Billwerk specifics

Problem/Motivation

The Billwerk module has no README.md but seems to have some undocumented speicifics additional to Subman. For example:

In class BillwerkSync there is:

self::MAP_SUBSCRIPTION_TYPE_ROLES => $this->utils->explodeValues($data['CustomFields']['cms_roles'] ?? ''),

Steps to reproduce

Proposed resolution

Write a README for the functionality and processes.

Remaining tasks

User interface changes

API changes

Data model changes