Add docs for contributor checklist before opening a contracts pull request

[Chucks1093]

Summary
Contributors opening pull requests against the contracts repo have no checklist to verify their change is ready for review. A short pre-PR checklist covering tests, formatting, docs, and safety considerations reduces back-and-forth in review.

Scope

• document a checklist that contributors should complete before opening a contracts pull request
• cover test coverage, clippy and fmt passes, storage safety, and event compatibility
• place the checklist in the contributing guide or as a pull request template

Acceptance Criteria

• the checklist covers at least five distinct items
• each item is actionable and specific to the contracts repo
• no code changes are required; this is a docs-only issue

Contributor coordination: if you pick this up, reach out in Telegram for updates or clarification: https://t.me/accesslayerorg

[newmattock]

@newmattock has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I'd love to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @newmattock to this issue.