To include the URL to this resource within my Addon (https://xenforo.com/community/resources/enable-etag-support-for-css.4082/updates) I first would have to create a resource. I took this steps:
#1 Created a resource with "External site: ..."
#2 Posted an update and told Resource Manager to upload my zip file.
#3 The addon is still showing "Download from external site"