Describe using calculates on readonly fields to display computed values to the user

This should go somewhere in https://docs.opendatakit.org/form-logic/#calculations

Start in the calculations section at https://docs.opendatakit.org/form-logic/#calculations, which the issue identifies as the likely location. Add a description of using calculates on readonly fields to display computed values to users; the documentation is done when that use case is clearly explained there.

Written by the indexing model from the issue text.