adds a simpler zip example with only 2 lists

As an infrequent zip user who forgets what it does, I found the zip documentation confusing. The description jumps too soon into the multiple-collection feature and side-cases without explaining the what zip is for. I think adding this simple example will help a bit.

http://ruby-doc.org/core-2.0.0/Enumerable.html#method-i-zip

Looks ok? Worth submitting a patch?