Add a note and pointer to Moose::Meta::Attribute::Native in
the reference to MooseX::AttributeHelpers in the cookbook's
Extending::Recipe1
metaclass extension. For example, the L<MooseX::AttributeHelpers>
distribution provides a new attribute metaclass that lets you delegate
behavior to a non-object attribute (a hashref or simple number).
+(MooseX::AttributeHelpers has been deprecated in favour of
+L<Moose::Meta::Attribute::Native>, but can still serve as an example).
A metaclass extension can be packaged as a subclass or a
role/trait. If you can, we recommend using traits instead of