Add an example on structs

subscriber = %Subscriber{}
=> %Subscriber{name: "", over_18: true, paid: false}

subscriber2 = %Subscriber{ name: "Dave" }
=> %Subscriber{name: "Dave", over_18: true, paid: false}

subscriber2.name
=> "Dave"

%Subscriber{ name: name } = subscriber2
=> %Subscriber{name: "Dave", over_18: true, paid: false}

name
=> "Dave"

subscriber3 = %Subscriber{ subscriber2 | name: "Mary" }
=> %Subscriber{name: "Mary", over_18: true, paid: false}