title: Is there a way to tell in the page that a service worker has recently upgraded?
body: <p>I have a SPA with a service worker.</p>
  <p>I don't want to force users to update, so usually the new worker updates in the background, but I notify users and they can click a button to message the worker to <code>skipWaiting</code>. When this happens I rely on <code>oncontrollerchange</code> to force any other tabs they have open to refresh.</p>
  <p>I want to show a link to release notes <em>after</em> the worker has upgraded, either due to refresh of all tabs or they force the refresh.</p>
  <p>However, I don't want to show these notes the first time they visit, or every time the service worker activates. If they don't read the release notes I don't want to keep nagging them about it.</p>
  <p>Is there an event or reliable design pattern I can use to tell (in the page) that the service worker has updated to a new version?</p>