1. You MUST have current dev firmware installed - this means a firmware date of 20161207 or later. If you don't feel like being treated like a Scratch Monkey*, DO NOT LOAD THIS FIRMWARE.
2. You're on your own. If the plugin devours your printer, destroys half your house, and your spouse leaves you...well you've got material for a good book I guess. Regardless, I'm not responsible.

3. Read README.md. It's not hard, it's on the front page of the repository. Then read it AGAIN. Maybe, just maybe, read it yet AGAIN.

4. If you don't have an HE280 equipped printer (or at least the fancy probe board), THIS WILL NOT WORK. The calibration code depends on responses generated by the SeeMeCNC fork of Repetier Firmware and the attached probe. That being said, this should work on any delta so equipped. (Upgraded v2, Orion, Eris, etc.)
Here's the link: https://github.com/geneb/OctoPrint-Delta-Calibration
You can install it via pip (see the readme!) (for you octopi users, pip is in /home/pi/oprint/bin/), or you can download the zip file and unpack it into /home/pi/.octoprint/plugins. I recommend you use pip as it's simply easier to deal with, and subsequent pip installation attempts will pull a new version if it's available.
g.
*https://en.wikipedia.org/wiki/Scratch_monkey