[guide] Document class-methods-use-this

This PR documents the currently enforced `class-methods-use-this` ESlint rule as discussed in #2129

README.md

@@ -1254,6 +1254,39 @@ Other Style Guides
     }
     ```
 
+  <a name="classes--methods-use-this"></a>
+  - [9.7](#classes--methods-use-this) Class methods should use `this` or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: [`class-methods-use-this`](https://eslint.org/docs/rules/class-methods-use-this)
+
+    ```javascript
+    // bad
+    class Foo {
+      bar() {
+        console.log('bar');
+      }
+    }
+
+    // good - this i used
+    class Foo {
+      bar() {
+        console.log(this.bar);
+      }
+    }
+
+    // good - constructor is exempt
+    class Foo {
+      constructor() {
+        // ...
+      }
+    }
+
+    // good - static methods aren't expected to use this
+    class Foo {
+      static bar() {
+        console.log('bar');
+      }
+    }
+    ```
+
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