Add User definition to example (close #459)

docsite/source/custom-types.html.md

@@ -42,6 +42,14 @@ valid[:invalid]
 `Types.Constructor` builds a new constructor type for the given class. By default uses the `new` method as a constructor.
 
 ```ruby
+class User
+  def initialize(attributes)
+    @attributes = attributes
+  end
+
+  def name = @attributes.fetch(:name)
+end
+
 user_type = Types.Constructor(User)
 
 # It is equivalent to User.new(name: 'John')
@@ -93,4 +101,4 @@ ListOfStrings = Types.Array(Types::String)
 ```ruby
 Callable = Types.Interface(:call)
 Contact = Types.Interface(:name, :phone)
-```
+```