title: Add item numbers to items in Magento Invoice PDF
body: <p>I am trying to do something that should be really simple, but I am stuck.</p>
  
  <p>I am trying to add serial numbers (1,2,3 etc) to the items in Magento's invoice pdf under a column called Serial Number.</p>
  
  <p>Any clues on how to do this?
  Thanks!</p>