title: How to know if service worker was updated in gatsby-plugin-offline
body: <p>I've installed <a href="https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-plugin-offline" rel="nofollow noreferrer">gatsby-plugin-offline</a>, It works fine, but I want to listen for sw update event so I can notify user that new version of app is available.</p>
  
  <p>Prior to this I used <a href="https://github.com/NekR/offline-plugin" rel="nofollow noreferrer">offline-plugin</a> Which I speculate is behinde gatsby's plugin? They have doc explaining how to achieve what I want here: <a href="https://github.com/NekR/offline-plugin/blob/master/docs/updates.md" rel="nofollow noreferrer">https://github.com/NekR/offline-plugin/blob/master/docs/updates.md</a> but I can't figure out how to get these events via gatsby, any suggestions guys?</p>