Add build status badge

Author: Philippus

README.md

@@ -1,5 +1,6 @@
 # scala-parser-combinators
 
+[![build](https://github.com/scala/scala-parser-combinators/workflows/test/badge.svg)](https://github.com/scala/scala-parser-combinators/actions/workflows/ci.yml?query=branch%3Amain)
 [<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)
 [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-parser-combinators_3.svg?label=latest%20release%20for%203"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-parser-combinators_3)