docs: iam model docstring

Issue: #13

src/iam/models.py

@@ -7,6 +7,15 @@ Models:
   - service_rel: ORM relationship over service_id FK
   - group_rel: ORM relationship backpops to Group.permission_rel
   - service_name: Calc property service_rel.name
+ - Group:
+  - id[PK], name, org_id[FK]
+  - user_rel: ORM relationship to User via UserGroups table
+  - org_rel: ORM relationship to Organisation using org_id FK
+  - permission_rel: ORM relationship to Permission via GroupPermissions table
+ - GroupPermissions:
+  - group_id[FK][PK], permission_id[FK][PK]
+ - UserGroups:
+  - org_id[FK][PK], user_id[FK][PK], group_id[FK][PK]
 """
 from sqlalchemy import Column, Integer, String, ForeignKey, UniqueConstraint
 from sqlalchemy.orm import relationship