Support plugins without rule docs

[bmish]

I have seen a few plugins that put all their rule docs in the top-level README.md:

• eslint-plugin-jsdoc

While such plugins can use --init-rule-docs to create new rule docs, they might prefer to keep all their rule docs in one doc file.

Possible ways we could achieve this:

• --path-rule-doc = README.md without the {name} placeholder would indicate that the docs are all in a single file
• --path-rule-doc = undefined to indicate the docs shouldn't be managed
• Or more explicit options