Add note that sbt 1.3.x is currently unsupported

Author: jrudolph

README.md

@@ -4,6 +4,8 @@
 
 Visualize your project's dependencies.
 
+**Note: sbt 1.3.x is currently not supported because by default sbt 1.3.x. [replaced Ivy with coursier](https://www.lightbend.com/blog/sbt-1.3.0-release). For the time being, if you want to use sbt-dependency-graph, either stay on sbt 1.2.x, or disable coursier (`ThisBuild / useCoursier := false`). Please also consider helping out implementing support for coursier in this plugin (see [#178](https://github.com/jrudolph/sbt-dependency-graph/issues/178)).**
+
 ## Usage Instructions
 
 sbt-dependency-graph is an informational tool rather than one that changes your build, so you will more than likely wish to