Added Directory badge (#27)

The badge will go to Directory with filter (author=vaadin&framework=polymer2)

README.md

@@ -1,4 +1,5 @@
 ![Bower version](https://badge.fury.io/bo/vaadin.svg)
+[![Published on Vaadin  Directory](https://img.shields.io/badge/Vaadin%20Directory-published-00b4f0.svg)](https://vaadin.com/directory/search?framework=Polymer%202&keyword=vaadin)
 [![Published on webcomponents.org](https://img.shields.io/badge/webcomponents.org-published-blue.svg)](https://www.webcomponents.org/author/vaadin)
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/vaadin/web-components?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)