Document common pitfall 'undefined method' in a bc app

[johrstrom]

I just closed this issue in the ondemand repo opened by a user and heaven knows I've ran into the same before.

Folks don't generally know ruby, or are even novice/beginner programmers and so are likely to be unaware of what the issue really is.

This ticket is to document the common pitfall that is using something in your submit.erb.yml that you haven't yet defined in your form.yml and getting an error message similar to what's below.

┆Issue is synchronized with this Asana task by Unito