This isn't a commonly used feature so it's probably largely undocumented.
You almost certainly don't need to create a Library class. These classes define how to translate data from a converter into the applications installed (e.g. blog, forums, downloads, etc.). Unless you are wanting to convert data from an external source into a new application within Invision Community, the library class would be unnecessary, because we already have libraries for all of our core applications.
A Converter class is needed for each type of content you wish to convert, and translates to the files you see under applications/convert/sources/Software/*. You'll see there are folders for each app (which are each defined by Library classes), and then underneath each folder is a class for each type of software supported. If you want to allow conversions of blog, calendar and forum content from "Some Random Made Up Forum v2.0", then you'll want to create software classes for Core, Blog, Calendar and Forum.
From there, you will need to define certain mapping methods that outline what is convertable, and you will need to create the methods that actually get called to perform the conversion. Your best option is to copy code from one of the existing software classes and modify it to suit your needs in most cases.