Add CHANGELOG

[jtdub]

Description

The project is at v3.4.1 with significant breaking changes from v2, but there is no CHANGELOG file documenting what changed between versions. Migration utilities exist in utils.py (e.g., load_hconfig_v2_options) but are not documented in a migration guide.

Proposed Improvement

1. Add a CHANGELOG.md following Keep a Changelog format
2. At minimum, document:
   • v3.0.0 breaking changes from v2 (Host class removal, driver system introduction, etc.)
   • Notable additions in v3.1-v3.4 (reporting, config views, future(), fast_load, etc.)
3. Optionally add a docs/migration-v2-to-v3.md guide that documents the migration path and links to the v2 utility functions in utils.py