docs: add loaderTopFiles.

Author: erasermeng

README.md

@@ -127,6 +127,25 @@ const webpackConfig = smp.wrap({
 });
 ```
 
+### `options.loaderTopFiles`
+
+Type: `Number`<br>
+Default: 0
+
+`loaderTopFiles` is used with `outputFormat: 'humanVerbose'`:
+
+```
+const smp = new SpeedMeasurePlugin({
+  outputFormat: 'humanVerbose',
+  loaderTopFiles: 10
+});
+```
+
+you can clearly see the most cost-effective loader files:
+
+![cost-effective-loader](cost-effective-loader.png)
+
+
 ### `options.granularLoaderData` _(experimental)_
 
 Type: `Boolean`<br>