Add some contribution guidelines.

CONTRIBUTING.md

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+# Contributing
+
+Everyone is welcome to suggest changes to the configured rules and settings, suggest new plugins, and update dependencies. The configuration is intended to be kept compatible with a variety of projects, which is one of the reasons why it's set up to work in a somewhat modular fashion. Do keep in mind that ultimately it's an opiniated configuration tailored to the needs of Fonto's developers.
+
+## General notes
+
+-   This package carefully defines exact (peer) dependency versions to guarantee compatability between all components. This is intentional. It might be relaxed in the future.
+-   The dependency versions in `package.json` should be kept in sync with the `README.md`.
+-   Although unnecessary for linting the configuration itself, this package has _all_ of its peer dependencies listed as development dependencies. There are two reasons for this:
+    1.  For [eslint-find-rules](https://www.npmjs.com/package/eslint-find-rules) to find unconfigured and deprecated rules (`npm run test`).
+    2.  To catch dependency version conflicts early on.
+
+## Adding/modifying rules
+
+-   After upgrading dependencies, run `npm run test` to find unconfigured and deprecated rules. Also check release notes for breaking changes, especially when upgrading to new major versions.
+-   Always check that a rule does not conflict with any of the used plugins.
+-   Keep in mind that a rule sometimes requires that another rule is turned off.
+-   Rules already defined by plugins and/or recommended configuration are omitted, unless overridden.
+-   Consider using `warn` for autofixable and codestyle-related rules. Use `error` for the more serious issues.
+-   Keep ordering and filetype overrides in mind when determining where to define a rule and when it should apply.
+
+## Rule annotations
+
+The following annotations are used in inline trailing comments:
+
+-   `@AUTOFIX`: for autofixable rules.
+-   `@PRETTIER`: for rules turned off by [eslint-config-prettier](https://github.com/prettier/eslint-config-prettier#special-rules) that are carefully enabled again.
+-   `@RECOMMENDED`: for rules (already) defined by recommended configuration.
+-   `@TODO`: for rules that need to be verified and/or configured.
+
+In addition, some rules benefit from an explanatory leading comment:
+
+-   `// Override: <reason>.`
+-   `// Turned off for <plugin name>.`
+
+## ESLint intricacies
+
+-   ESLint plugins should _always_ be peer dependencies for shared configuration due to the way ESLint resolves import locations of plugins.
+    -   See: https://github.com/eslint/eslint/issues/3458
+    -   See: https://github.com/eslint/rfcs/tree/main/designs/2019-config-simplification
+-   `overrides` defined in ESLint configuration _deeply merge_ with any other matching configuration. This is not unambiguously documented by ESLint.
+    -   See: https://github.com/eslint/eslint/issues/13585