Update docs to add note about back-compat

Also add some very basic upgrading notes for the early adopters.

I also updated the tutorial to use client objects.

cc @danielgtaylor @kyleknap 
I think the main reason is because I was not intending for this to be a changelog.  It's meant to represent breaking changes that may affect existing users.  I'd imagine once we hit a 1.0 release, we should no longer have any upgrade notes, as everything going forward will be backwards compatible.

I think we should eventually have a CHANGELOG, but I only intend for this section to be relevant until we hit 1.0.