The Guide for the Migration Tool is pretty incomplete when it comes to workflow stuff (unless you're ok with deploying all workflow elements wholesale every time). There are a number of things that can be deployed individually.
I remember seeing someone's post about the details of them, but couldn't find it anymore. Basically they all need to be dot qualified by the object they belong to (similar to custom fields), then listed under the correct type names to be used by the package.xml. For instance, a Field Update looks like this: