message: docs: add link to upgrade helper in keeping backstage updated page
diff: @@ -41,7 +41,10 @@ For this reason, any changes made to the template are documented along with
  upgrade instructions in the
  [changelog](https://github.com/backstage/backstage/blob/master/packages/create-app/CHANGELOG.md)
  of the `@backstage/create-app` package. We recommend peeking at this changelog
  -for any applicable updates when upgrading packages.
  +-for any applicable updates when upgrading packages. As an alternative, the
  +[Backstage Upgrade Helper](https://backstage.github.io/upgrade-helper/) provides
  +a consolidated view of all the changes between two versions of Backstage. You
  +can find the current version of your Backstage installation in `backstage.json`.
  ## More information on dependency mismatches
  