sync react readme with upstream

Author: JasonBoy

react/README.md

@@ -280,6 +280,25 @@
   // good
   <div />
   ```
+  - 避免使用数组的index来作为属性`key`的值，推荐使用唯一ID. ([为什么?](https://medium.com/@robinpokorny/index-as-a-key-is-an-anti-pattern-e0349aece318))
+
+  ```jsx
+  // bad
+  {todos.map((todo, index) =>
+    <Todo
+      {...todo}
+      key={index}
+    />
+  )}
+
+  // good
+  {todos.map(todo => (
+    <Todo
+      {...todo}
+      key={todo.id}
+    />
+  ))}
+  ```
 
 ## Parentheses 括号