Add badge for scala 2.13

Author: Philippus

README.md

@@ -3,6 +3,7 @@
 [<img src="https://img.shields.io/travis/scala/scala-parser-combinators.svg"/>](https://travis-ci.org/scala/scala-parser-combinators)
 [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-parser-combinators_2.11.svg?label=latest%20release%20for%202.11"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-parser-combinators_2.11)
 [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-parser-combinators_2.12.svg?label=latest%20release%20for%202.12"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-parser-combinators_2.12)
+[<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-parser-combinators_2.13.svg?label=latest%20release%20for%202.13"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-parser-combinators_2.13)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/scala/scala-parser-combinators)
 
 ### Scala Standard Parser Combinator Library