Added notes about nested MultipleInput to the docs

Author: unclead

UPGRADE.md

@@ -8,6 +8,14 @@ if you want to upgrade from version A to version C and there is
 version B between A and C, you need to following the instructions
 for both A and B.
 
+Upgrade from 1.2 to 1.3
+-----------------------
+
+- The mechanism of customization configuration by using index placeholder was changed in scope of implementing support of nested `MultipleInput`
+If you customize configuration by using index placeholder you have to add ID of widget to the placeholder.
+For example, `multiple_index` became `multiple_index_question_list`
+
+
 Upgrade from version less then 1.1.0
 ------------------------------------
 

docs/tips.md

@@ -149,4 +149,36 @@ as in the example below:
 
 ##Embedded MultipleInput widget <a id="embedded"></a>
 
-TBD
+You can use nested `MultipleInput` as in the example below:
+
+```
+echo MultipleInput::widget([
+    'model' => $model,
+    'attribute' => 'questions',
+    'attributeOptions' => $commonAttributeOptions,
+    'columns' => [
+        [
+            'name' => 'question',
+            'type' => 'textarea',
+        ],
+        [
+            'name' => 'answers',
+            'type'  => MultipleInput::class,
+            'options' => [
+                'attributeOptions' => $commonAttributeOptions,
+                'columns' => [
+                    [
+                        'name' => 'right',
+                        'type' => MultipleInputColumn::TYPE_CHECKBOX
+                    ],
+                    [
+                        'name' => 'answer'
+                    ]
+                ]
+            ]
+        ]
+    ],
+]);
+```
+
+But in this case you have to pass `attributeOptions` to the widget otherwise you will not be able to use ajax or client side validation of data.