🧭 Umbrella: document submodule workflow (dev vs main)

### Description
Clarify the official workflow for Rix submodules:

- `dev` branch for active development  
- `main` branch for stable releases  
- versioning policy between the umbrella and individual modules  

### Tasks
- [ ] Add a **Submodule workflow** section to the README  
- [ ] Explain when and why to modify `.gitmodules`  
- [ ] Document the release strategy (`rix v0.x` ↔ `modules v1.x`) 