Add information on how to install vagrant-cachier

Readme.md

@@ -29,10 +29,11 @@ wget http://files.vagrantup.com/packages/87613ec9392d4660ffcb1d5755307136c06af08
 sudo dpkg -i vagrant_x86_64.deb
 ```
 
-Install a vagrant plugin to keep the virtualbox guest additions up-to-date.
+Install a vagrant plugin to keep the virtualbox guest additions up-to-date, as well as one for keeping a package cache.
 
 ```bash
 vagrant plugin install vagrant-vbguest
+vagrant plugin install vagrant-cachier
 ```
 
 ## To start building