Readme. Add "Dirty API" section

Author: andrykonchin

README.md

@@ -475,6 +475,10 @@ used: `attributes_before_type_cast` and `read_attribute_before_type_cast`.
 
 There is `<name>_before_type_cast` method for every field in a model as well.
 
+### Dirty API
+
+Dynamoid supports Dirty API which equvalents to [Rails 5.2 `ActiveModel::Dirty`](https://api.rubyonrails.org/v5.2/classes/ActiveModel/Dirty.html). There is only one limitation - change in place of field is't detected automatically.
+
 ## Usage
 
 ### Object Creation