document globalOptions

`globalOptions` is useful when a babel plugin needs non-serializable options (such as functions, callbacks), thus it's worth mentioning in the README.

Author: anilanar

README.md

@@ -80,6 +80,23 @@ module: {
   ]
 }
   ```
+  
+  or by using global options:
+
+  ```javascript
+module: {
+  loaders: [
+    {
+      test: /\.js$/,
+      exclude: /(node_modules|bower_components)/,
+      loader: 'babel-loader'
+    }
+  ]
+},
+babel: {
+    presets: ['es2015']
+}
+  ```
 
   This loader also supports the following loader-specific option: