Update spree_active_shipping part

[mrpollo]

there has been some major changes to how spree_active_shipping works and I think we should update this section, I'll try to send a PR outlining the major changes and probably some common pitfalls I've had to deal with and I know some other people keep having.

[radar]

@mrpollo Are you still going to work on this PR?

[mrpollo]

Yeah, there are lots of changes, I already started some work on the README on that project, I will try to get it worked on this week 😁