Add a number of downloads badge

Author: ignisf

README.md

@@ -1,5 +1,6 @@
 # libv8
 [![Gem Version](https://badge.fury.io/rb/libv8.svg)](http://badge.fury.io/rb/libv8)
+[![Number of downloads](http://ruby-gem-downloads-badge.herokuapp.com/libv8?type=total)](https://rubygems.org/gems/libv8)
 [![Build Status](https://travis-ci.org/cowboyd/libv8.svg?branch=master)](https://travis-ci.org/cowboyd/libv8)
 [![Join the chat at https://gitter.im/cowboyd/therubyracer](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/cowboyd/therubyracer?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Code Triagers Badge](https://www.codetriage.com/cowboyd/libv8/badges/users.svg)](https://www.codetriage.com/cowboyd/libv8)