Update README - Add Group Workspace role

@paullizer is already working on this.

Since Feb 6, 2026.

## Roles and Permissions

1. **Group Owner**
- **Creation:** Automatically assigned to the user who creates the group.
- **Membership Management:** Can add/remove members, approve membership requests, and manage roles (promote users to admin or document manager).
- **Group Settings:** Can rename the group, delete the group, and transfer ownership if desired.
- **Highest Authority:** Has all permissions of a Group Admin plus full control over the group lifecycle.
2. **Group Admin**
- **Membership Management:** Can add/remove members, approve membership requests, and assign roles (promote members to Group Admin or Document Manager).
- **Group Settings:** None.
- **Cannot Delete Group**
3. **Group Document Manager**
- **Document-Focused Role:** Responsible for organizing and managing documents within the group.
- **No Membership Management**
4. **Group User**
- **Basic Member:** Can participate in group activities (list group documents, chat using group documents, and read group documents).
- **Membership Requests:** Can request to join a group.
- **No Administrative Privileges:** Cannot add/remove other users or change any group settings.