docs: explain migration from coverage run to pytest-cov

[xuu33030]

Summary

• Add a README section for coverage.py users explaining why switching from coverage run -m pytest to pytest --cov can change imports and reported coverage.
• Recommend installing the package rather than depending on the working directory on sys.path, and point to the existing subprocess migration note.
• Add the changelog entry and author attribution requested by CONTRIBUTING.

Closes #482.

Verification

• In a clean temporary environment, coverage run -m pytest reported CWD on sys.path, whereas pytest --cov did not (same test file, both passed).
• sphinx-build -b html -n -W docs <scratch-output>: passed before and after the change with no warnings.
• git diff --check: passed.

This is a documentation-only change; the project's full tox matrix was not run. AI-assisted contribution; I reviewed the documentation, checked the behavior with an isolated reproducer, and ran the documentation build.