[audit/critical] Add CONTRIBUTING.md with entry format spec

[Displacedtex]

Priority: Critical

Problem

README says "You can take pull requests and contribute" but there is no CONTRIBUTING.md, no PR template, and no entry format specification. The community can't reliably help maintain the list without rules.

Recommendation

Create CONTRIBUTING.md covering:

• How to propose a new entry (category, format, link style).
• What qualifies (original projects vs meta-lists — both? just originals?).
• No-affiliate-links rule (unless explicitly disclosed).
• Dead-link reporting workflow (point to issue template once created).
• Required checks before submitting a PR (link tested, category chosen).

Acceptance criteria

• CONTRIBUTING.md at repo root.
• README links to it.

Part of the repo audit backlog. Related: #8 (governance files).