docs: add 'Adding a New Domain' section to CONTRIBUTING.md

[sqllocks]

Summary

Adds an explicit "Adding a New Domain" section to CONTRIBUTING.md, between Running Tests and Adding a Custom Strategy.

Motivation: Issue #1 invites contributors to add a small new domain, but CONTRIBUTING.md doesn't describe the scaffold flow. This PR closes that gap so first-time contributors who land on the repo via search or stars (not via the issue tracker) get the same guidance.

What changed

• New section: "Adding a New Domain" (~25 lines)
• Covers: scaffold copy, schema definition, calibration source citation, smoke test, docs entry, PR tagging
• Lists suggested domain ideas (bookstore, events, library, conference, gym, telecom, gaming, logistics)
• Links to Discussions + Issue Add a small new domain (e.g. bookstore or events) #1 for contributors who want to talk it through first

Test plan

• Markdown renders correctly
• All internal links resolve
• No changes to code

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