Document modularization for future authentication enhancements

[dezidizon]

Document modularization for future authentication enhancements.

[adamkorynta]

Modularization is documented in the README.md: https://github.com/opendcs/rest_api/pull/186/files#diff-1a58cf3a903823820e954ae726c4bb6ca5ad2a7002bd893db166aed5468c267e which describes interfaces and configurations to change for additional authorization types.