[guide] [react] add note that defaultProps must always be provided for non-required props

Author: ljharb

react/README.md

@@ -337,6 +337,35 @@
   ))}
   ```
 
+  - Always define explicit defaultProps for all non-required props.
+
+  > Why? propTypes are a form of documentation, and providing defaultProps means the reader of your code doesn’t have to assume as much. In addition, it can mean that your code can omit certain type checks.
+
+  ```jsx
+  // bad
+  function SFC({ foo, bar, children }) {
+    return <div>{foo}{bar}{children}</div>;
+  }
+  SFC.propTypes = {
+    foo: PropTypes.number.isRequired,
+    bar: PropTypes.string,
+    children: PropTypes.node,
+  };
+
+  // good
+  function SFC({ foo, bar }) {
+    return <div>{foo}{bar}</div>;
+  }
+  SFC.propTypes = {
+    foo: PropTypes.number.isRequired,
+    bar: PropTypes.string,
+  };
+  SFC.defaultProps = {
+    bar: '',
+    children: null,
+  };
+  ```
+
 ## Refs
 
   - Always use ref callbacks. eslint: [`react/no-string-refs`](https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/no-string-refs.md)