[tei-council] addition of <availability>

[Syd Bauman]

CORRECTION (thanks, Martin)

Kevin was indeed in favor of #3, but I wrote it wrong. Corrected: 

  3. <availability> should be permitted as a direct child of
     <imprint> (presumably along with model.noteLike, idno,
     model.ptrLike, relatedItem, and citedRange after the ultimate
     <monogr> or <series>), but not as a descendant of <series>,
     <monogr>, or <analytic>.