Add Scaladex badge to supply info on supported Scala versions

Badge introduced with scalacenter/scaladex#660

Author: rtyley

README.md

@@ -2,3 +2,5 @@ scala-git
 =========
 
 _Scala veneer for JGit_
+
+[![scala-git artifacts](https://index.scala-lang.org/rtyley/scala-git/scala-git/latest-by-scala-version.svg)](https://index.scala-lang.org/rtyley/scala-git/scala-git/)