If you download the nuget package it exists a readme.txt file in the packages folder for this nuget package. In this text file you can find what this package is and how to use it.

Thanks for the tip Fredrik!

EPiServer.Packaging.Converter

Overwiew
============

EPiServer.Packaging.Converter.exe is a conversion tool that can be used to convert previous AddOn formats to
"true" nuget format to make them possible to install through Visual Studio as other nuget packages. The new
format will be supported by AddOn UI as well meaning the converted package can either be installed through
Visual Studio or through AddOn UI.

Installation
------------------------
EPiServer.Packaging.Converter is a tools package so it will not install anything to the Visual Studio project.
Instead is the conversion tool available in \packages\EPiServer.Packaging.Converter<version>\tools folder.

Result
------------------------
A converted package will have all content files relocated so they are prefixed with a path "/modules/<packageid>/"
for public AddOns or "/modules/_protected/<packageid>/" for proteced AddOns. Resources for addons can optionally be
packaged and delivered on site through a zip file. Zip packaging can be enabled through argument as below.
It will also convert assembly dependencies (which was supported previously) to nuget dependencies. It will automatically
handle "EPiServer" (will be converted to EPiServer.CMS.Core), "CMS" (will be converted to EPiServer.CMS.UI) and
"Shell" (will be converted to EPiServer.CMS.UI), custom dependencies can be mapped, se usage section.
It will also output a generated .nuspec file that you can use (and possibly modify) to build the package yourself.

Usage
------------------------
The conversion tool is a command line tool. The required argument '-s' is the path to the Addon package (.nupkg) that
should be converted. The path should be an absolute path. The argument '-o' is an optional argument
which specifies the outpath where the converted package will be created. If not given it will replace the original
AddOn package (the original will be saved in parallel with extension ".org").
There is an optional argument '-d' to handle custom assembly mappings to nuget package ids. The format should be
"assembly1=package1,assembly2=package2". The argument '-z true' can be given if content files should be packaged
into a zip archive.