Add example/documentation for LeanZipFile

[DanielOaks]

This is an awesome module, thanks for writing it! Using it to access a zip file that's a few gigabytes, brings the access speeds way down.

I did get a bit stuck on how to actually use LeanZipFile though, had to dig into source code to work it out. It might be good to add an example in the readme or some documentation of how to use that class, since the interface does differ from the standard ZipFile.