title: Add space between SliverAppBar and SliverList in Flutter
body: <p>I have a SliverAppBar and below it, a SliverList.</p>
  
  <p>The first SliverList item is too close to the SliverAppBar. I'd like to add some spacing between, whether via a bottom margin below the SliverAppBar or above the SliverList.</p>
  
  <p>How can this be done?</p>