Provide example in documentation for custom datamodel

[t-b]

I'm proposing to include a minimal MWE for a biber.conf which adds a custom datamodel.

Something like

<?xml version="1.0" encoding="UTF-8"?>
<config>
  <datamodel>
    <constants>
      <constant name = "nothing"></constant>
    </constants>
    <entrytypes>
      <entrytype>person</entrytype>
    </entrytypes>
    <fields>
        <field fieldtype="list" datatype="literal">name</field>
    </fields>
      <entryfields>
        <entrytype>person</entrytype>
          <field>name</field>
      </entryfields>
    <multiscriptfields>
      <field>name</field>
    </multiscriptfields>
    <constraints/>
  </datamodel>
</config>

In case you have some CI this should also be in a separate file and checked against the current relaxng schema.

I would also propose to change the schema so that constants and multiscriptfields can be empty.

[plk]

Addressed in DEV branch.