docs: correct README filename in contributor guide

[travisfortney]

Hi! I'm getting some practice with small open-source contributions and noticed that the contributor guide still points to README.rst. The repository uses README.md, so this corrects the filename and adds the required history note.

Local validation on macOS:

• CPython 3.10–3.15 and PyPy 3.10 test suites passed, with the existing skips and expected failures.
• Ruff lint/format checks and Python 3.14 typing examples passed.
• All 253 doctests and the strict HTML documentation build passed; both changed pages were checked in the rendered output.
• Wheel/sdist validation passed, with 100% combined statement coverage across the test runs.

The optional ujson dependency needed a local linker setting to build under PyPy on macOS. No runtime code, dependency versions or project configuration changes are included.

Thanks for taking a look!