Document the location of and constraints on the internal database for backup/restore purposes

A straightforward approach to creating a backup of all of ZKAPAuthorizer's state is to follow the SQLite3-recommended backup process of ZKAPAuthorizer's SQLite3 database. All state is there. To restore from backup, copy the backed up database into the correct place in the filesystem and start a ZKAPAuthorizer-enabled Tahoe-LAFS node. That's all.

If ZKAPAuthorizer promises the database will remain at its current path for a while and points out the SQLite3 recommendations for backup then it becomes possible for a third-party to manage backup/recover of ZKAPAuthorizer state.