[dx] Create CHANGELOG.md and document v0.1.0 release

[MarvyNwaokobia]

What needs to be done

Create CHANGELOG.md at the repo root documenting what was built in v0.1.0.
Follow the Keep a Changelog format: https://keepachangelog.com

Format:

# Changelog

All notable changes to shielded-contracts are documented here.
Format: [Keep a Changelog](https://keepachangelog.com/en/1.1.0/)

## [Unreleased]

## [0.1.0] — 2026-05-09

### Added
- CommitmentPool contract: deposit, withdraw, get_root, is_nullifier_spent, pause, unpause
- MerkleTree: persistent Soroban storage, depth 20, 1M commitment capacity
- NullifierSet: double-spend prevention using persistent storage
- Groth16Verifier: structural BN254 verifier scaffold (full pairing check in progress)
- ComplianceRegistry: encrypted viewing key grant/revoke/retrieve
- ShieldedProxyBlend: scaffold for Blend lending protocol integration
- Error enum: 8 documented error codes across all contracts
- Event structs: Deposited, Withdrawn, Paused, Unpaused
- Unit tests: 5 passing tests covering initialize, root, nullifier, pause/unpause
- GitHub Actions CI: Rust format, Clippy, test pipeline

Definition of done

• CHANGELOG.md exists at repo root
• v0.1.0 entry complete and accurate
• Unreleased section present for future changes
• Committed and pushed

Estimated time

20 min

Required knowledge

Markdown. No code knowledge needed.