Update README.md

Author: gaearon

README.md

@@ -282,6 +282,25 @@ export default connect(
 )(SandwichShop);
 ```
 
+## Injecting a Custom Argument
+
+Since 2.1.0, Redux Thunk supports injecting a custom argument using the `withExtraArgument` function:
+
+```js
+const store = createStore(
+  reducer,
+  applyMiddleware(thunk.withExtraArgument(api))
+)
+
+// later
+function fetchUser(id) {
+  return (dispatch, getState, api) => {
+    // you can use api here
+  }
+}
+```
+
+
 ## License
 
 MIT