Update README.md

README.md

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 KVS: Abstract Chain Database
 ============================
 [![Build Status](https://travis-ci.org/synrc/kvs.svg?branch=master)](https://travis-ci.org/synrc/kvs)
+[![Hex pm](http://img.shields.io/hexpm/v/kvs.svg?style=flat)](https://hex.pm/packages/kvs)
 
 Features
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