Update README.md

doc: trim self-promotion and over-defensive phrasing; tighten §1, §5, §6

doc: rephrase Limitations note now that manifest date_range matches parquet exactly

fix(manifests): correct AEMO forecast date_range to actual parquet extent (2025-01-27 to 2026-02-01)

fix(manifests): correct AEMO 5min date_range to actual parquet extent (2024-09-18 to 2026-02-12)

Initial release: parquet traces + manifests + grid cases

initial commit