Clarify versioning policy and expectations around breaking changes

## Overview
The `base/account-sdk` repository provides API usage examples and reference documentation, but it does not clearly explain how versioning is handled or what guarantees (if any) exist around backward compatibility and breaking changes.

## Problem Description
Developers integrating the SDK into production systems may be unsure about:
- how often breaking changes are expected
- whether semantic versioning is strictly followed
- how to safely plan upgrades
- which releases may require code changes

This uncertainty can make long-term maintenance and upgrade planning more difficult.

### Proposed Solution
- Add a short section to the README (e.g. “Versioning & Compatibility”).
- Clearly describe:
- whether semantic versioning is used
- how breaking changes are communicated
- recommended practices for upgrading between major/minor versions
- Keep the guidance high-level and non-prescriptive.