update readme.

Author: opiepj

README.md

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+[![npm version](https://badge.fury.io/js/webpack-shell-plugin.svg)](https://badge.fury.io/js/webpack-shell-plugin)
+![](https://reposs.herokuapp.com/?path=1337programming/webpack-shell-plugin)
+[![npm](https://img.shields.io/npm/dm/webpack-shell-plugin.svg)]()
 # Webpack Shell Plugin
 
 This plugin allows you to run any shell commands before or after webpack builds. This will work for both webpack and webpack-dev-server.