Add Instructions how to use it from an "foreign" Vagrantfile

[tknerr]

Right now we only describe how to use it with the example Vagrantfile in this repo, but not how to use it from a Vagrantfile that lives in another repo (e.g. an "infrastructure repository")

Some options:

• using the vagrant-toplevel-cookbooks plugin
• using from the supermarket (does this respect the berkshelf git deps?!?)
• using a tarball uploaded to github releases for this repo with all deps included (just an idea - is that possible at all?!?)

[tknerr]

http://berkshelf.com/#packaging-cookbooks looks good for the 3rd option