Add Javadoc badge

Author: dekmm

README.md

@@ -1,6 +1,7 @@
 Java-Runtime-Compiler
 =====================
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/net.openhft/compiler/badge.svg)](https://maven-badges.herokuapp.com/maven-central/net.openhft/compiler)
+[![Javadoc](https://javadoc.io/badge2/net.openhft/compiler/javadoc.svg)](https://javadoc.io/doc/net.openhft/compiler)
 
 This takes a String, compiles it and loads it returning you a class from what you built.  
 By default it uses the current ClassLoader.  It supports nested classes, otherwise builds one class at a time.