strong parameter example for hashes with integer keys.

This replicates the situation described in
rails/strong_parameters#114

guides/source/action_controller_overview.md

@@ -290,6 +290,20 @@ parameters:
 params.require(:author).permit(:name, books_attributes: [:title, :id, :_destroy])
 ```
 
+Hasesh with integer keys are treated differently and you can declare
+the attributes as if they were direct children. You get this kind of
+parameters when you use `accepts_nested_attributes_for` in combination
+with a `has_many` association:
+
+```ruby
+# To whitelist the following data:
+# {"book" => {"title" => "Some Book",
+#             "chapters_attributes" => { "1" => {"title" => "First Chapter"},
+#                                        "2" => {"title" => "Second Chapter"}}}}
+
+params.require(:book).permit(:title, chapters_attributes: [:title])
+```
+
 #### Outside the Scope of Strong Parameters
 
 The strong parameter API was designed with the most common use cases