title: How to ask user permission for receiving notifications?
body: <p>I am using Swift 3.0 and I want the user to be able to click on a button to trigger the alert box that requests his permission for using notifications.</p>
  
  <p>I am surprised not to find more information about that.</p>
  
  <p>I would like to support iOS 9.0 as well as 10.</p>
  
  <p>What is the way to trigger this ask-for-permission alert box again ?</p>