[guide] add eslint rule reference for prefer-object-spread

Author: 1pete

README.md

@@ -306,7 +306,7 @@ Other Style Guides
     ```
 
   <a name="objects--rest-spread"></a>
-  - [3.8](#objects--rest-spread) Prefer the object spread operator over [`Object.assign`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.
+  - [3.8](#objects--rest-spread) Prefer the object spread operator over [`Object.assign`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted. eslint: [`prefer-object-spread`](https://eslint.org/docs/rules/prefer-object-spread)
 
     ```javascript
     // very bad