docs: Added the rules for hooks_avoid_within_class.

README.md

@@ -21,6 +21,7 @@ The currently available lint rules are as follows:
 | LintRule  | Description | Quickfix |
 | ------------- | ------------- | ------------- |
 | [hooks_avoid_nesting](#hooks_avoid_nesting)  | You should use Hooks only inside the build method of a Widget. | |
+| [hooks_avoid_within_class](#hooks_avoid_within_class)  | Hooks must not be defined within the class. | |
 | [hooks_name_convention](#hooks_name_convention)  | DO always prefix your hooks with use, https://pub.dev/packages/flutter_hooks#rules. | |
 | [hooks_extends](#hooks_extends)  | Using Hooks inside a Widget other than HookWidget or HookConsumerWidget will result in an error at runtime. | ✅ |
 | [hooks_unuse_widget](#hooks_unuse_widget)  | If you are not using Hooks inside of a Widget, you do not need HookWidget or HookConsumerWidget. | ✅ |
@@ -118,6 +119,30 @@ Widget build(BuildContext context) {
 }
 ```
 
+### hooks_avoid_within_class
+
+Defining Custom Hooks within a class mixes the characteristics of class and method, leading to potentially complex code.
+
+**Bad**
+
+```dart
+class TestHelper {
+  const TestHelper._();
+
+  static void useEffectOnce(Dispose? Function() effect) { // ❌
+    useEffect(effect, const []);
+  }
+}
+```
+
+**Good**:
+
+```dart
+void useEffectOnce(Dispose? Function() effect) { // ⭕
+  useEffect(effect, const []);
+}
+```
+
 ### hooks_name_convention
 
 **DO always prefix your hooks with use.**