Added error for attempted ivy-style publication

README.md

@@ -1,6 +1,6 @@
 # sbt-github-packages [![Build Status](https://travis-ci.com/djspiewak/sbt-github-packages.svg?branch=master)](https://travis-ci.com/djspiewak/sbt-github-packages)
 
-Configures your project for publication to the [GitHub Package Registry](https://help.github.com/en/articles/about-github-package-registry) using its Apache Maven support. Note that you probably shouldn't use this with plugins, only libraries. Also provides some convenience functionality for *depending* upon artifacts which have been published to the Package Registry.
+Configures your project for publication to the [GitHub Package Registry](https://help.github.com/en/articles/about-github-package-registry) using its Apache Maven support. Note that GitHub Packages *exclusively* supports maven-style publication; using Ivy style will result in a warning. Also provides some convenience functionality for *depending* upon artifacts which have been published to the Package Registry.
 
 ## Usage
 

src/main/scala/sbtghpackages/GitHubPackagesPlugin.scala

@@ -59,11 +59,18 @@ object GitHubPackagesPlugin extends AutoPlugin {
   val packagePublishSettings = Seq(
     publishTo := {
       val log = streams.value.log
+      val ms = publishMavenStyle.value
       val back = for {
         owner <- githubOwner.?.value
         repo <- githubRepository.?.value
       } yield "GitHub Package Registry" at s"https://maven.pkg.github.com/$owner/$repo"
 
+      back foreach { _ =>
+        if (!ms) {
+          sys.error("GitHub Packages does not support Ivy-style publication")
+        }
+      }
+
       back orElse {
         GitHubPackagesPlugin synchronized {
           if (!alreadyWarned) {