Adding shield.io twitter count/tweet button.

Author: seth-reeser

README.md

@@ -2,6 +2,7 @@ Vagrant Host Manager
 ====================
 
 [![Gem](https://img.shields.io/gem/dt/vagrant-hostmanager.svg)](https://rubygems.org/gems/vagrant-hostmanager)
+[![Twitter](https://img.shields.io/twitter/url/https/shields.io.svg?style=social)](https://twitter.com/intent/tweet?text=Check%20out%20this%20awesome%20Vagrant%20plugin%21&url=https%3A%2F%2Fgithub.com%2Fsmdahlen%2Fvagrant-digitalocean&hashtags=vagrant%2Cdigitalocean&original_referer=)
 
 `vagrant-hostmanager` is a Vagrant 1.1+ plugin that manages the `/etc/hosts`
 file on guest machines (and optionally the host). Its goal is to enable