Add machine-readable dist index

[brson]

For scripts to discover the contents of the dist archives we probably need an index in a standard markdown format. Since all of my scripts are bourne shell I'd prefer it be something more easily parsable than json, or at least if it's json that it be formatted in a way that can be grepped easily.

[nagisa]

cc me

[brson]

Should probably include a metadata version number in it for future proofing.

[Aatch]

For something this simple, JSON is probably overkill. Some delimiter-separated value is probably the best way.

[brson]

This is done.