Add a note about the "download" attribute for anchors

HTML specifies an attribute to suggest to the client the filename to use
when saving to the local filesystem, something like
"Content-disposition: attachment; filename" but per HTML element, which
sounds good, see:
http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#downloading-resources

This would be an interesting feature for DDL, but browser support is
very limited as for now.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>