docs(readme): add javadoc badge

Author: javier-godoy

README.md

@@ -2,7 +2,7 @@
 [![Stars on vaadin.com/directory](https://img.shields.io/vaadin-directory/star/extended-login-add-on.svg)](https://vaadin.com/directory/component/extended-login-add-on)
 [![Build Status](https://jenkins.flowingcode.com/job/ExtendedLogin-addon/badge/icon)](https://jenkins.flowingcode.com/job/ExtendedLogin-addon)
 [![Maven Central](https://img.shields.io/maven-central/v/org.vaadin.addons.flowingcode/extended-login)](https://mvnrepository.com/artifact/org.vaadin.addons.flowingcode/extended-login)
-
+[![Javadoc](https://img.shields.io/badge/javadoc-00b4f0)](https://javadoc.flowingcode.com/artifact/org.vaadin.addons.flowingcode/extended-login)
 
 # Extended Login Add-on