The proposal document (in PDF) proposes a way to manage third-party packages (extension libraries) for scsh.
Version 1.3.0 of the library (released on 2005-08-20), which has to be installed to install scsh packages. It includes the above documentation.
Older versions are still available:
These kits provide what's necessary to install third-party Scheme libraries as scsh packages. They have to be unarchived in the same directory as the original library.