docs: README pre-commit and endpoint guide

[ayoub3bidi]

Documents local pre-commit setup and the Flyway-to-test flow for new endpoints.

Made with Cursor

[ayoub3bidi]

Branch cleanup

Reset to main and cherry-picked only the README commit (README.md only — 1 file).

Verification

Check	Result
Docs-only PR — no runtime code changes	—
docker compose run mercury_linter	All checks passed
Fresh Docker API stack	Flyway OK, logs clean
GET /v1/health	200
GET / (Swagger)	200
POST /v1/token	200

README adds Pre-commit setup and How to add a new endpoint (Flyway → model → schema → controller → route → tests).

Ready to merge.