Add an example on how to read a zipfile

Hi,

This PR adds an example to the README on how to read a zip archive, showing a few common use cases. I was surprised by how much searching through the documentation I had to do to achieve this, so I'm guessing this will help many people to get started. Especially because with zipruby, rubyzip < 1 and rubyzip > 1 there are now three different Zip API's, which can be confusing.

Best regards,
Arne