Add the option :immutable

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Update documentation for :immutable

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Documentation now declares ruby syntax

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>