Add general section on encoding licence information

[cmil]

We provide licence information in several places, for instance in the digital source of a play, the licence terms of an individual TEI document or an entire corpus. Maybe we should add a general section on how to encode this information.

[cmil]

In the guidelines section for Digital Source we give the following example:

dracor-schema/dracor.odd

Lines 531 to 542 in c6c8262

	<egXML xmlns="http://www.tei-c.org/ns/Examples">
	<bibl type="digitalSource">
	<name>TextGrid Repository</name>
	<idno type="URL">http://www.textgridrep.org/textgrid:11f78.0</idno>
	<availability>
	<licence>
	<ab>CC-BY-3.0</ab>
	<ref target="http://creativecommons.org/licenses/by/3.0/de/legalcode">Lizenzvertrag</ref>
	</licence>
	</availability>
	</bibl>
	</egXML>

The licence markup seems to be unnecessarily verbose to me and with "Lizenzvertrag" contains boilerplate text that we don't actually need. I would suggest the following more concise markup instead:

<licence target="http://creativecommons.org/licenses/by/3.0/de/legalcode">CC-BY-3.0</licence>

Also see the examples for <licence> in the TEI guidelines.

[cmil]

See also #107 (comment).

[juliajbeine]

I second that.

[cmil]

PR #164 changes all the <licence> examples in the ODD to the simplified markup. I have also adjusted the templates in the Oxygen framework (dracor-org/dracor-oxygen-framework@902e583) as well as the transformations for EngDraCor (dracor-org/engdracor#75) and FreDraCor (dracor-org/fredracor#38). This may be sufficient for now. More specific instructions might be the outcome of #163.