original pointers from @marineam at coreos/docs#222 (comment)
btrfs adding devices and rebalancing: https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices#Adding_new_devices
btrfs rebalance command: http://askubuntu.com/a/412307 essence of that: don't use -mconvert=raid0 since that would mean "use raid0 for metadata also" which is not advisable
running fdisk in non-interactive mode: http://xmodulo.com/how-to-run-fdisk-in-non-interactive-batch-mode.html
additional docs by coreos that already point in the right direction and that are probably a candidate for introducing explicit information about getting up and running with RAID: https://coreos.com/docs/cluster-management/debugging/btrfs-troubleshooting/
Here's another piece of documentation I hadn't seen: https://coreos.com/docs/cluster-management/debugging/btrfs-troubleshooting/#adding-a-new-physical-disk
Maybe it's even easier and the "create partition" step isn't even required. That hadn't worked for me when trying to add the disk to btrfs.