Docs: shortest path to add a scoring rule

[cucuwang]

Why

docs/methodology.md already defines evidence classes and the fixture contract, but a new contributor has to reverse-engineer src/core/rules.ts and fixtures/v0.6/rule-corpus.ts to land a first change.

Ask

Add a short section (README or CONTRIBUTING, not a new scoring rule) that states the minimum path:

1. Pick an evidence class (A–D) from docs/methodology.md.
2. Add the rule in src/core/rules.ts with a stable id.
3. Add one positive, one negative, and one false-positive fixture in fixtures/v0.6/rule-corpus.ts.
4. Note JSON / score-contract impact.

Do not claim the rule improves ranking, AI Overviews, or ChatGPT/Perplexity citation.

Out of scope

Implementing a new rule in the same PR, unless it is only an example already covered by the corpus.

Pointers

• docs/methodology.md (Validation policy)
• src/core/rules.ts
• fixtures/v0.6/rule-corpus.ts
• CONTRIBUTING.md