add section about ${} reference variables

[lindsay-stevens]

There is a section on the ODK docs but not XLSForm.org. It should mention key things like:

• the referenced name can be any question, group, or repeat on the survey sheet; or instanceID to get the unique identifier for that submission.
• when a form uses them, the referenced name has to be unique anywhere in the form. The workaround in that case is to write the XPath string that would have been inserted.
• if the referenced name is prefixed with last-saved# (e.g. ${last-saved#age}) then the token will be resolved to a reference to the value of that name in the previously saved record.
• reference variables can be used on the sheets: settings, entities, survey, and choices; in most columns except for name/type columns.