The documentation and first place to go is ruby.sketchup.com and for general Ruby features ruby-doc.org. For listing files on the file system, you need to use the File/Directory API.
Let's first talk about what you want to create / what capabilities it needs. Normally you don't create an installer for its own sake. Furthermore, "installers" are a distribution concept specific to Windows, and you don't necessarily need them elsewhere. For example in SketchUp, you create an "extension package" (.rbz) which is distributed through the store and users usually don't get in touch with it as a file.