Starter: write contributor setup notes for integration PRs

[sridharkalaibala]

Parent issue: #34

Goal

Add beginner-friendly setup notes for contributors who want to work on app, connector, ecommerce, or webhook tasks.

Suggested files to inspect

• README.md
• api/package.json
• frontend/
• connectors/
• .github/workflows/
• Issue App and connector platform: reviewable integrations without weakening the offline POS core #34

Acceptance criteria

• Docs explain how to install dependencies for the API/frontend/connector area being changed.
• Docs include focused test commands contributors should run before opening a PR.
• Docs say contributors must not use real customer data or real API keys in tests.
• Docs explain that Community Edition tasks must run without Posnic Cloud unless the issue explicitly says otherwise.
• The PR links back to App and connector platform: reviewable integrations without weakening the offline POS core #34.

Validation

• Run git diff --check.
• Follow the documented commands on a clean checkout or clearly document any missing prerequisite.

Out of scope

• Rewriting the whole README.
• Adding secrets or production config.