Update DeveloperGuide.md

docs/DeveloperGuide.md

@@ -132,7 +132,8 @@ How the parsing works:
 
 The `Model` component,
 
-* stores the address book data i.e., all `Person` objects (which are contained in a `UniquePersonList` object).
+* stores the current address book data i.e., all `Person` objects (which are contained in a `UniquePersonList` object).
+* stores the history of address book data in `AddressBookList`.
 * stores the currently 'selected' `Person` objects (e.g., results of a search query) as a separate _filtered_ list which is exposed to outsiders as an unmodifiable `ObservableList<Person>` that can be 'observed' e.g. the UI can be bound to this list so that the UI automatically updates when the data in the list change.
 * stores a `UserPref` object that represents the user’s preferences. This is exposed to the outside as a `ReadOnlyUserPref` objects.
 * does not depend on any of the other three components (as the `Model` represents data entities of the domain, they should make sense on their own without depending on other components)