Add ESLint custom rules documentation (#1947)

## Summary

Adds comprehensive documentation for the custom ESLint rules directory,
specifically documenting the `no-use-client-in-server-files` rule that
enforces proper usage of server components in React Server Components
architecture.

## What's Included

- Why the rule exists (Shakapacker 9.3.0+ compatibility)
- Clear examples of correct and incorrect usage
- Auto-fix capability documentation
- Testing instructions
- Guidance for adding new custom rules in the future

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added comprehensive documentation for a custom ESLint rule, including
usage examples, configuration guidelines, auto-fix capabilities, and
testing procedures.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

eslint-rules/README.md

@@ -0,0 +1,110 @@
+# Custom ESLint Rules
+
+This directory contains custom ESLint rules specific to React on Rails.
+
+## Rules
+
+### `no-use-client-in-server-files`
+
+Prevents the `'use client'` directive from being used in `.server.tsx` and `.server.ts` files.
+
+#### Why This Rule Exists
+
+Files ending with `.server.tsx` are intended for server-side rendering in React Server Components (RSC) architecture. The `'use client'` directive forces webpack to bundle these files as client components, which creates a fundamental contradiction and causes errors when using React's `react-server` conditional exports.
+
+This issue became apparent with Shakapacker 9.3.0+, which properly honors `resolve.conditionNames` in webpack configurations. When webpack resolves imports with the `react-server` condition, React's server exports intentionally omit client-only APIs like:
+
+- `createContext`, `useContext`
+- `useState`, `useEffect`, `useLayoutEffect`, `useReducer`
+- `Component`, `PureComponent`
+- Other hooks (`use*` functions)
+
+#### Examples
+
+❌ **Incorrect** - Will trigger an error:
+
+```typescript
+// Component.server.tsx
+'use client';
+
+import React from 'react';
+
+export function MyComponent() {
+  return <div>Component</div>;
+}
+```
+
+✅ **Correct** - No directive in server files:
+
+```typescript
+// Component.server.tsx
+import React from 'react';
+
+export function MyComponent() {
+  return <div>Component</div>;
+}
+```
+
+✅ **Correct** - Use `'use client'` in client files:
+
+```typescript
+// Component.client.tsx or Component.tsx
+'use client';
+
+import React, { useState } from 'react';
+
+export function MyComponent() {
+  const [count, setCount] = useState(0);
+  return <div>Count: {count}</div>;
+}
+```
+
+#### Auto-fix
+
+This rule includes an automatic fixer that will remove the `'use client'` directive from `.server.tsx` files when you run ESLint with the `--fix` option:
+
+```bash
+npx eslint --fix path/to/file.server.tsx
+```
+
+#### Related
+
+- **Issue:** [Shakapacker #805 - Breaking change in 9.3.0](https://github.com/shakacode/shakapacker/issues/805)
+- **Fix PR:** [React on Rails #1896](https://github.com/shakacode/react_on_rails/pull/1896)
+- **Commit:** [86979dca - Remove 'use client' from .server.tsx files](https://github.com/shakacode/react_on_rails/commit/86979dca)
+
+#### Configuration
+
+This rule is automatically enabled in the React on Rails ESLint configuration at the `error` level. It's defined in `eslint.config.ts`:
+
+```typescript
+plugins: {
+  'react-on-rails': {
+    rules: {
+      'no-use-client-in-server-files': noUseClientInServerFiles,
+    },
+  },
+},
+
+rules: {
+  'react-on-rails/no-use-client-in-server-files': 'error',
+  // ... other rules
+}
+```
+
+## Testing
+
+To run tests for the custom rules:
+
+```bash
+node eslint-rules/no-use-client-in-server-files.test.cjs
+```
+
+## Adding New Custom Rules
+
+To add a new custom ESLint rule:
+
+1. Create the rule file in this directory (use `.cjs` extension for CommonJS)
+2. Create a corresponding test file (e.g., `rule-name.test.cjs`)
+3. Import and register the rule in `eslint.config.ts`
+4. Add documentation to this README