Update README.md documentation

Adds a mention and an example of default values for attributes.

README.md

@@ -99,6 +99,20 @@ If we try to pass an invalid value to the initializer, we’ll get an error. Und
 def first_name = @name.split(/\s/).first
 ```
 
+Attributes can have default values.
+
+```ruby
+class Task
+  extend Literal::Attributes
+
+  attribute :timeout, Integer, default: 60
+  attribute :options, Hash, default: proc { { notify_on_completion: true } }
+  attribute :states, Array, default ["initialized", "started", "finished"].freeze
+end
+```
+
+Defaults must be immutable so strings, hashes and arrays must be either frozen or wrapped in a Proc.
+
 The type-checking is really designed for the public interface. Internally, I think it’s good practice to reference the instance variables directly. You can always use the writers (which are private by default) if you need to do type-checking, but that’s not usually necessary. We’re not trying to make the application type-safe. We can’t do that without a complete type system. What we’re doing is adding some helpful checks to the main public interfaces.
 
 ## `Literal::Enum`