goreleaser: on release, display a description on GitHub

I'm not sure it works, as I couldn't test locally. These changes are
expected to update the description when a release is performed, for
example here: https://github.com/koyeb/koyeb-cli/releases/tag/v3.4.0

Author: brmzkw

.goreleaser.yaml

@@ -47,6 +47,12 @@ archives:
 
 release:
   prerelease: auto
+  header: |
+    ## The best version ever of Koyeb CLI is here!
+
+    Welcome to this new release!
+    Check the changelog in [CHANGES.md](https://github.com/koyeb/koyeb-cli/blob/master/CHANGES.md) for more details.
+    For upgrade instructions, see [README.md](https://github.com/koyeb/koyeb-cli/blob/master/README.md).
 
 dockers:
   - goos: linux