Add Scaladex badge to supply info on supported Scala versions

Badge introduced with scalacenter/scaladex#660

README.md

@@ -4,7 +4,14 @@ play-secret-rotation
 _Rotating your [Application Secret](https://www.playframework.com/documentation/2.6.x/ApplicationSecret)
 on an active cluster of Play app servers_
 
-More docs on how to do this:
+[![play-secret-rotation artifacts](https://index.scala-lang.org/guardian/play-secret-rotation/play-v28/latest-by-scala-version.svg)](https://index.scala-lang.org/guardian/play-secret-rotation/play-v28/)
+
+[![play-secret-rotation artifacts](https://index.scala-lang.org/guardian/play-secret-rotation/play-v27/latest-by-scala-version.svg)](https://index.scala-lang.org/guardian/play-secret-rotation/play-v27/)
+
+[![play-secret-rotation artifacts](https://index.scala-lang.org/guardian/play-secret-rotation/play-v26/latest-by-scala-version.svg)](https://index.scala-lang.org/guardian/play-secret-rotation/play-v26/)
+
+
+Detailed docs on how to use this library:
 
 * [Using AWS Parameter Store](aws-parameterstore/README.md)
 * ...but not with _AWS Secrets Manager_, because