Write CONTRIBUTING.md 'Adding a new vendor' guide

[bryanwintermute]

Summary

receipt_print.py is brand-agnostic, but every NV-config CLI in the repo today is Rongta-specific (nv_config.py, ethernet_config.py, papersave_config.py, blackmark_config.py, other1_config.py, plus the rongta_config.py dispatcher). If a contributor shows up wanting to add Star / Bixolon / Xprinter / Epson NV-config support, there's no documented path for what to do or where files should land.

Proposal

Add a new section to CONTRIBUTING.md: "Adding a new vendor".

It should cover, end-to-end, what we did for Rongta so a contributor can follow it on their own printer without re-deriving the workflow:

1. The capture rig pattern — Wine + a custom logging CUPS backend + USB-IP-export from a Pi. Link to docs/wine-cups-backend-recovers-nv-bytes.md which already has the recipe.
2. The 4-clicks diffing methodology — all-off / only-X / only-Y / only-Z gives complete bit-mapping for any group of boolean NV flags.
3. Where to put the new files. For now: top-level, prefixed with the vendor name (e.g., star_config.py). Note that a vendors/<name>/ layout is on the roadmap (#N, vendor-namespace-refactor) — if that lands first, new vendors go under vendors/<name>/.
4. What tests look like — the dry-run byte-equality fixtures in tests/. We want the same contract for every vendor: emitted bytes are stable across refactors.
5. PR template / labels — a vendor:<name> label per new vendor; what to include in the PR description (printer model, USB id, vendor-tool version captured against).
6. Out-of-scope guardrails — sanitize / markdown / receipt-rendering work belongs in receipt_print.py, not in any vendor module. The vendor modules are only for NV-config and any other proprietary side-channels.

Doubles as an anchor

The README "Help wanted" block (WiFi tab, Bluetooth tab, UDP discovery, factory-reset checksum reversal, Voiceless-volume verification, cutter-stats BULK-IN response, etc.) currently links to a single help-wanted issue template. Once this guide exists, those help-wanted items can link to specific sections of it.

Acceptance

• New section in CONTRIBUTING.md, ~150-300 lines.
• README's "Help wanted" callout updated to link to the new section.
• (Optional, if natural while writing): a per-new-vendor issue template at .github/ISSUE_TEMPLATE/.

Related

• Pairs with Reorganize into vendors/<name>/ layout (v0.3.0, on-demand) #5 (vendor-namespace-refactor) but should ship first. We document the seams before we move the files.