Updated readme and other documentation to reflect new API changes.

This brings the README.md and other docs inline with the current plugin, 0.9.11 (codename: extra-dogey)
"in order to migrate your data to the new service"

Maybe also be explicit here about the migration strategy -- you should start by looking to see if there's any data using SavedGames. And if there isn't, then use the CloudSave data.