add javadoc link

Author: dwickern

README.md

@@ -3,6 +3,7 @@
 
 [![Build](https://github.com/dwickern/scala-nameof/workflows/build/badge.svg)](https://github.com/dwickern/scala-nameof/actions)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.github.dwickern/scala-nameof_2.13/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.github.dwickern/scala-nameof_2.13)
+[![javadoc](https://javadoc.io/badge2/com.github.dwickern/scala-nameof_2.13/javadoc.svg)](https://javadoc.io/doc/com.github.dwickern/scala-nameof_2.13/latest/com/github/dwickern/macros/NameOf.html)
 
 Get the name of an variable, function, class member, or type as a string--at compile-time!