[doc] add storages doc to index.

Author: makasim

Resources/doc/index.md

@@ -2,6 +2,7 @@
 
 * [Get it started](get_it_started.md)
 * [Simple purchase examples](simple_purchase_examples.md).
+* [Storages](storages.md).
 * [Purchase done action](purchase_done_action.md).
 * [Custom action usage](custom_action_usage.md).
 * [Custom api usage](custom_api_usage.md).