Add Hex badget to README.

README.md

@@ -1,4 +1,5 @@
 [![Build Status](https://travis-ci.org/ostinelli/syn.svg?branch=master)](https://travis-ci.org/ostinelli/syn)
+[![Hex pm](https://img.shields.io/hexpm/v/syn.svg)](https://hex.pm/packages/syn)
 
 
 # Syn