amended README.md

README.md

@@ -33,7 +33,7 @@ Vagrant.configure("2") do |config|
   end
 end
 ```
-And then run `vagrant up --provider=dustcloud`.
+And then run `vagrant up --provider=dustcloud`. Instead of `token_file`, you can also put the token in an environment variable called `DUSTCLOUD_TOKEN`.
 
 ## Development
 To work on the `vagrant-dustcloud` plugin, clone this repository out, and use