Update 0.4 release notes

docs/docs/the-marten-project/release-notes/0.4.md

@@ -12,6 +12,31 @@ Crystal 1.7, 1.8, and 1.9.
 
 ## New features
 
+### Multi table inheritance
+
+It is now possible to define models that inherit from other concrete models (ie. non-abstract models). In this situation, each model can be used/queried individually and has its own associated database. The framework automatically defines a set of "links" between each model that uses multi table inheritance and its parent models in order to ensure that the relational structure and inheritance hierarchy are maintained.
+
+For example:
+
+```crystal
+class Person < Marten::Model
+  field :id, :big_int, primary_key: true, auto: true
+  field :first_name, :string, max_size: 100
+  field :last_name, :string, max_size: 100
+end
+
+class Employee < Person
+  field :company_name, :string, max_size: 100
+end
+
+employee = Employee.filter(first_name: "John").first!
+employee.first_name # => "John"
+```
+
+All the fields defined in the `Person` model can be accessed when interacting with records of the `Employee` model (despite the fact that the data itself is stored in distinct tables).
+
+You can read more about this new kind of model inheritance in [Multi table inheritance](../../models-and-databases/introduction#multi-table-inheritance).
+
 ### Schema handler callbacks
 
 Handlers that inherit from the base schema handler - [`Marten::Handlers::Schema`](pathname:///api/dev/Marten/Handlers/Schema.html) - or one of its subclasses (such as [`Marten::Handlers::RecordCreate`](pathname:///api/dev/Marten/Handlers/RecordCreate.html) or [`Marten::Handlers::RecordUpdate`](pathname:///api/dev/Marten/Handlers/RecordUpdate.html)) can now define new kinds of callbacks that allow to easily manipulate the considered [schema](../../schemas/introduction) instance and to define logic to execute before the schema is validated or after (eg. when the schema validation is successful or failed):