Addon .nfo files look like this:

(supertux-addoninfo
  (id "octo-levels")
  (version 1)
  (type "worldmap")
  (title "Octo's Levels")
  (author "Octo")
  (license "GPL 2+ / CC-by-sa 3.0"))

The id is a identifier for this addon, it has to be unique across all addons, as it is used to compare the addons with new ones from other sources to find updates. It is recomment to use something like "{author}-{title}". The id must be all lowercase and only contain characters of the set "[a-z][0-9]-", underscore is not allowed as it is used for the version number.

The version number is a simple integer, it should be increased each time the addon is changed.

The type gives an indication of what is contained within the addon, valid values are "worldmap", "world", "levelset". At the moment this is only a description for the user and doesn't have any impact on how the addon is handled.

The .nfo file itself needs to be stored in the top-level directory of the addon and should be named by the unique id of the addon, i.e. "/octo-levels.nfo" in this example.