message: Add link to Demos site in README
diff: @@ -24,6 +24,8 @@ This blog post provides more examples of how Backstage is used inside Spotify:
  https://labs.spotify.com/2020/03/17/what-the-heck-is-backstage-anyway/
  +https://backstage.io/demos
  +
  ## Project roadmap
  We created Backstage about 4 years ago. While our internal version of Backstage has had the benefit of time to mature and evolve, the first iteration of our open source version is still nascent. We are envisioning three phases of the project and we have already begun work on various aspects of these phases:
  