Document schematron validation for TL

[rockivist]

Need to account for schematron validation somewhere in TL. Probably need @tcatapano to address this.

[kerstarno]

Issue review in preparation to upcoming EAD subteam call on 23 September:

While the TL includes a general mention of the Schematron schema ("A Schematron schema is also available to provide further validation functionality for EAD instances, imposing data constraints that either cannot be expressed in DTD, Relax NG, and W3C, or were intentionally removed from the schemas by TS-EAD due to challenges of maintaining code lists outside of our control or to allow alternative data sources or patterns."), I am not sure that already constitutes for having the schematron validation documented.

@karinbredenberg and @fordmadox - might this rather be an issue for the Schema subteam?

[karinbredenberg]

Perhaps this one can be closed and the Schematron description that is created but not published needs to be keept in the same place as #508

[kerstarno]

During their meeting on 28 October, EAD team of TS-EAS decided to assign this issue along with issues #501 and #508 to @fordmadox as team lead of the Schema team to indicate ownership being with that team.

Given that Schematron is only used with EAD3 at the moment, there remains, however, a close connection to the EAD team in finalising these issues.