Consider documenting external rules configured by a plugin's configs

[bmish]

We could generate a separate "external rules" list in the README that would have a table of external rules that a plugin's configs enabled/disable.

Examples:

• if the recommended config of some plugin disables a particular eslint core rule, then it could be mentioned in this separate external rules table
• eslint-plugin-square has configs that control rules from a bunch of different plugins
• The example in this issue: Consider splitting out a generalized testing plugin jest-community/eslint-plugin-jest#1289 (comment)

Low priority for now but theoretically something we could implement.

External rules are likely less important to document vs. internal rules.

Related:

• Support plugins with foreign rules #257