How to build a multi-page form in phoenix?

For https://github.com/healthlocker/healthlocker @katbow & Co. have built a multi-page user registration form as requested by the client team.
See: https://github.com/healthlocker/healthlocker/blob/master/web/controllers/user_controller.ex
We need to get the _learning_ out of this work and make it generic so _everyone_ knows how to build a multi-page form.
This will be _especially_ useful for our https://github.com/dwyl/feedback project where we want people to _first_ leave feedback and then allow them to associate an email address with that feedback so they can receive updates. @jackcarlisle if you want to work with @katbow to get this documented with a small demo app it would be _amaze_! thanks!
`how-to-build-multi-page-forms.md` thoughts?