New: Update Default Ignore Patterns

designs/2019-update-default-ignore-patterns/README.md

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+- Start Date: 2019-11-28
+- RFC PR: (leave this empty, to be filled in later)
+- Authors: Toru Nagashima ([@mysticatea](https://github.com/mysticatea))
+
+# Update Default Ignore Patterns
+
+## Summary
+
+This proposal changes the default ignore patterns to include `node_modules` in subdirectories and to exclude `.eslintrc.js`.
+
+## Motivation
+
+- In monorepo style development, there are `node_modules`s in subdirectories. But the current default ignore patterns don't include it, so we have to configure ESLint with `node_modules` to ignore it.
+- Because ESLint ignores dotfiles, it doesn't lint `.eslintrc.js` by default. We have to configure ESLint with `!.eslintrc.js` to lint it.
+
+## Detailed Design
+
+This proposal changes the default ignore patterns:
+
+- (as-is) `.*`
+- (new) `!.eslintrc.js`
+- (change) `/node_modules/*` → `/**/node_modules/*`
+- (as-is) `/bower_components/*`
+
+Therefore, ESLint lints `.eslintrc.js` and ignores `node_modules` in subdirectories.
+
+### How about the config files of other tools?
+
+For example, Babel has `.babelrc.js` and VuePress has `.vuepress/config.js`. ESLint ignores such files as dotfiles by default.
+
+This proposal doesn't change this behavior because we don't want to have allowlist/denylist for all tools in the world. Instead, we can have unignoring patterns in the `ignorePatterns` property of shareable configs for your tools.
+
+## Documentation
+
+- It needs an entry in the migration guide for 7.0.0 (or another major release) because of breaking changes.
+- It needs to update the "[.eslintignore](https://eslint.org/docs/user-guide/configuring#eslintignore)" section to explain the new default ignore patterns.
+
+## Drawbacks
+
+This proposal doesn't increase maintenance cost because it just updates a pattern list.
+
+If somebody wants to lint `node_modules` in subdirectories, they have to configure ESLint to unignore those. But I guess that it's rarer than wanting to ignore `node_modules` in subdirectories.
+
+## Backwards Compatibility Analysis
+
+This is a breaking change.
+
+- ESLint may report more warnings on `.eslintrc.js`.
+- ESLint may stop with fatal errors (all files are ignored) in the `node_modules` of subdirectories.
+
+## Alternatives
+
+We can use the `ignorePatterns` of shareable configs to configure and reuse this kind of matter.
+
+## Related Discussions
+
+- https://github.com/eslint/eslint/issues/1163 - ignore node_modules folder by default
+- https://github.com/eslint/eslint/issues/10089 - .babelrc.js should not be ignored by default
+- https://github.com/eslint/eslint/issues/10341 - do not ignore files started with `.` by default