docs: improve project onboarding README

[ChuanWeis]

Summary

• Replaces the default React Native README with a Flash POS-specific project overview.
• Adds setup, local development, supported hardware, project structure, docs, and contributor guidance.
• Adds three lightweight SVG screenshots/reference visuals for the README.
• Adds a CONTRIBUTING guide with local workflow and PR checklist.

Closes #49.

Validation

• Checked README relative links locally.
• Checked SVG files for valid opening/closing tags.
• Ran git diff --check.
• Ran corepack yarn install --frozen-lockfile successfully, then removed node_modules.

Existing project checks that were attempted but did not pass before/after this docs-only change:

• corepack yarn test --runInBand currently fails on existing test/dependency config issues (rewardSlice, missing @testing-library/react-native, and react-redux ESM transform).
• corepack yarn lint currently reports existing source lint errors unrelated to these README/docs changes.