add gh login instructions to install requirements (#58)

Author: maltesander

release/README.adoc

@@ -48,6 +48,7 @@ This script requires https://github.com/mikefarah/yq (not to be confused with ht
 ### gh (github client)
 
 This script requires https://github.com/cli to be installed.
+You have to be logged in when running the `post-release.sh` script. The easiest way is to have a local env variable `GH_TOKEN` set to a classic token created via https://github.com/settings/tokens with all `repo` and all `admin:org` scopes or follow the login instructions here https://cli.github.com/manual/gh_auth_login.
 
 ## Usage