title: Adding an existing site column to a custom list
body: <p>I think I'm going mad - this seemed like an easy thing to do but I can't find any info on it at all. </p>
  
  <p>I have created a Custom List and added 4 columns to it. Created By and Modified By are already in the list but hidden from the view. </p>
  
  <p>I want to add a Date Modified column (which is a built in field) to this Custom List. How do I do this programmatically?</p>