The adafruit-pi-externalroot-helper script is mostly an implementation of the process outlined in paulv's HOWTO: Move the filesystem to a USB stick on the Raspberry Pi forums, with additional help from the following sources:

• The Arch Linux wiki, GUID Partition Table for what a GPT is and some parted commands.
• GUID Partition Table, Wikipedia.
• Booting from an external USB drive, from the Raspberry Pi StackExchange.
• Booting a Raspberry Pi reliably from USB in the presence of multiple USB drives, by Stefan Krastanov.
• Speed up your Pi by booting to a USB flash drive, by Sam Hobbs.
• This comment on root= in init/do_mounts.c in the Linux source tree.

While the steps here are fairly straightforward once you put them together, this can be a finicky process. If you run into any problems, please consider filing an issue on the GitHub project.