Implement contract state migration notes

[El-swaggerito]

Description

Difficulty: Hard

Type: Documentation

Summary

Document state migration and upgrade assumptions for the Savings Vault contract.

Current Behaviour

Contract state structure changes may be made without explaining upgrade or migration impact.

Expected Behaviour

The repo should document current storage assumptions, compatibility risks, and review requirements for storage-changing PRs.

Suggested Implementation

Create storage documentation, migration notes, and PR review checklist.

Files or Areas Likely Affected

• docs/
• README.md
• contracts/
• src/

Acceptance Criteria

• Storage assumptions are documented.
• State migration risks are explained.
• Storage-changing PR review guidance is added.
• Compatibility risks are listed.
• Tests expected for storage changes are documented.
• README links to migration notes.

Additional Notes

This supports safer contract evolution.