docs: add Spanish README

[webbrain-one]

Adds README.es-ES.md alongside the original documentation.

[aa8y]

Thanks for the contribution! Since this PR was opened, the English README has been restructured on master — it went from one ~320-line file to a short overview plus per-topic pages — so a single translated README.es-ES.md of the old layout no longer lines up with the repo.

The current structure is:

• README.md — short overview: intro, the dataset support matrix (now 6 engines including DuckDB), quick start, tag conventions, and links
• postgres/README.md, mysql/README.md, cockroach/README.md, sqlite/README.md, duckdb/README.md — per-engine details
• docs/building.md, docs/testing.md, docs/ATTRIBUTION.md — build internals, testing, and dataset licenses

Could you rework this PR to follow that layout? Translating just the new short README.md (as README.es-ES.md, same naming you used) based on current master is a fine scope — the support matrix and dataset list have changed substantially. If you want to go further, per-engine translations would use the same suffix pattern (e.g. postgres/README.es-ES.md). Please keep the English files untouched.

Happy to review once it's rebased onto current master.