Parent ticket: #101016

Summary

README in rule_registry plugin is out of date and needs to reflect all the recent changes:

• made to the API of RuleDataService
• made to RBAC and RBAC-related endpoints
• in the schema
• in the behaviour that are important to know

If documenting some of these things would be too much of an effort (e.g. documenting full schema), we could create a separate ticket for that, add a placeholder section to the README and put a link to the ticket.