Add note about external dependencies to docs, formatting

Author: danielamorse

README.md

@@ -30,7 +30,7 @@ const DrupalLibrariesPlugin = require('drupal-libraries-webpack-plugin');
 
 module.exports = {
   plugins: [
-  	new DrupalLibrariesPlugin()
+    new DrupalLibrariesPlugin()
   ],
 };
 ```
@@ -45,6 +45,14 @@ You can explicitly add a Drupal library dependency to module by using a special
 require('@drupal(core/jquery)')
 ```
 
+Drupal libraries must be added as [external dependencies](https://webpack.js.org/configuration/externals/) in your webpack configuration:
+
+```js
+externals: {
+  '@drupal(drupal/core)': 'Drupal'
+}
+```
+
 ### Configuration
 
 #### `path`