  #### Writable nested serialization.
  
  REST framework 2.x attempted to automatically support writable nested serialization, but the behavior was complex and non-obvious. Attempting to automatically handle these case is problematic:
  
  * There can be complex dependancies involved in order of saving multiple related model instances.
  * It's unclear what behavior the user should expect when related models are passed `None` data.
  * It's unclear how the user should expect to-many relationships to handle updates, creations and deletions of multiple records.
  
  Using the `depth` option on `ModelSerializer` will now create **read-only nested serializers** by default. To use writable nested serialization you'll want to declare a nested field on the serializer class, and write the `create()` and/or `update()` methods explicitly.
  
 
</s> remove This change also means that we no longer use the `.full_clean()` method on model instances, but instead perform all validation explicitly on the serializer. This gives a cleaner seperation, and ensures that there's no automatic validation behavior on `ModelSerializer` classes that can't also be easily replicated on regular `Serializer` classes.
 </s> add This change also means that we no longer use the `.full_clean()` method on model instances, but instead perform all validation explicitly on the serializer. This gives a cleaner separation, and ensures that there's no automatic validation behavior on `ModelSerializer` classes that can't also be easily replicated on regular `Serializer` classes. </s> remove * Some data, such as many-to-many relationships, cannot be added to the object instance until after it has been saved. This type of data needed to be hidden in some undocumentated state on the object instance, or kept as state on the serializer instance so that it could be used when `.save()` is called.
 </s> add * Some data, such as many-to-many relationships, cannot be added to the object instance until after it has been saved. This type of data needed to be hidden in some undocumented state on the object instance, or kept as state on the serializer instance so that it could be used when `.save()` is called. </s> remove The `.restore_object()` method is now replaced with two seperate methods, `.create()` and `.update()`.
 </s> add The `.restore_object()` method is now replaced with two separate methods, `.create()` and `.update()`. </s> remove You can also still use the `many=True` argument to serializer classes. It's worth noting that `many=True` argument transparently creates a `ListSerializer` instance, allowing the validation logic for list and non-list data to be cleanly seperated in the REST framework codebase. 
 </s> add You can also still use the `many=True` argument to serializer classes. It's worth noting that `many=True` argument transparently creates a `ListSerializer` instance, allowing the validation logic for list and non-list data to be cleanly separated in the REST framework codebase.  </s> remove We now have a better seperation, with seperate `required` and `allow_none` arguments.
 </s> add We now have a better separation, with separate `required` and `allow_none` arguments.
* There can be complex dependencies involved in order of saving multiple related model instances.
  #### Writable nested serialization.
  
  REST framework 2.x attempted to automatically support writable nested serialization, but the behavior was complex and non-obvious. Attempting to automatically handle these case is problematic:
  
* There can be complex dependencies involved in order of saving multiple related model instances.
  * It's unclear what behavior the user should expect when related models are passed `None` data.
  * It's unclear how the user should expect to-many relationships to handle updates, creations and deletions of multiple records.
  
  Using the `depth` option on `ModelSerializer` will now create **read-only nested serializers** by default. To use writable nested serialization you'll want to declare a nested field on the serializer class, and write the `create()` and/or `update()` methods explicitly.
  
keep keep keep keep replace keep keep keep keep keep
 Minor: fix spelling and grammar, mostly in 3.0 announcement 
docs/topics/3.0-announcement.md