add prerelease description

Author: jjw24

appveyor.yml

@@ -64,6 +64,7 @@ deploy:
   - provider: GitHub
     repository: Flow-Launcher/Prereleases
     release: v$(prereleaseTag)
+    description: 'This is the early access build of our upcoming release. All changes contained here are reviewed, tested and stable to use.\n\nSee our [release](https://github.com/Flow-Launcher/Flow.Launcher/pulls?q=is%3Aopen+is%3Apr+label%3Arelease) Pull Request for details.\n\nFor latest production release visit [here](https://github.com/Flow-Launcher/Flow.Launcher/releases/latest)\n\nPlease report any bugs or issues over at the [main repository](https://github.com/Flow-Launcher/Flow.Launcher/issues)'
     auth_token:
       secure: ij4UeXUYQBDJxn2YRAAhUOjklOGVKDB87Hn5J8tKIzj13yatoI7sLM666QDQFEgv
     artifact: Squirrel Installer, Portable Version, Squirrel nupkg, Squirrel RELEASES