[tei-council] Addition to HD re schemaSpec in encodingDesc

[Martin Holmes]

Hi all,

Working on this ticket:

<http://purl.org/TEI/FR/1650195>

I've added <schemaSpec> to model.encodingDescPart, which makes it 
available in <encodingDesc>, and I've added a little section to the 
Header chapter to cover its usage:

<http://teijenkins.hcmc.uvic.ca:8080/job/TEIP5-Documentation/lastSuccessfulBuild/artifact/Guidelines-web/en/html/HD.html#HDSCHSPEC>

Take a look if you have a chance, and let me know if there's anything 
that should be changed or added.

Cheers,
Martin