guide about using bootstrap less files

docs/guide/bootstrap-widgets.md

@@ -45,3 +45,22 @@ framework features. All widgets belong to `\yii\bootstrap` namespace:
 - NavBar
 - Progress
 - Tabs
+
+
+Using the .less files of Bootstrap directly
+-------------------------------------------
+
+If you want to include the [Bootstrap css directly in your less files](http://getbootstrap.com/getting-started/#customizing)
+you may need to disable the original bootstrap css files to be loaded.
+You can do this by setting the css property of the `BootstrapAsset` to be empty.
+For this you need to configure the `assetManagner` application component as follows:
+
+```php
+    'assetManager' => [
+        'bundles' => [
+            'yii\bootstrap\BootstrapAsset' => [
+                'css' => [],
+            ]
+        ]
+    ]
+```