Footnotes are not too hard, just use <fn>, but note that for the time being, what can go inside a footnote is very, very restricted. 1  For instance, you can't put a <p> (and thus you can't put lists) inside a footnote. Also, no displayed math via <me>. There's a lot of care being taken because of the prospect of footnotes inside footnotes inside footnotes.
Because of the restrictions on footnotes, it is important to keep them short. A good alternative for longer things that are somewhat digressional is the aside, which comes in three flavors: <aside>, <biographical>, <historical>. Each of these allows an optional title and then a variety of tags such as <p>, <figure>, and <sidebyside> (and many more).