bug#1318: document footnote-mode

[jidanni]

In footnote.el
     (defun footnote-mode (&optional arg)
       "Toggle footnote minor mode.
[Say here in lieu of better documentation:] To get started, play around
with these keys:
     \\<message-mode-map>
     key                binding
     ---                -------
Remove blank line here.
     \\[Footnote-renumber-footnotes]
     \\[Footnote-goto-footnote]
     \\[Footnote-delete-footnote]
     \\[Footnote-cycle-style]
     \\[Footnote-back-to-message]
     \\[Footnote-add-footnote]
Reorder the above with the most common operations first. Also
hardwiring them here often looses track of the complete list. Perhaps
just mention the most often used and say that is all you are mentioning.