Explain how perfplat.dev is supposed to work

This was the first thing that I didn't understand since it didn't work as advertised. I grepped the source and found `if Vagrant.has_plugin? "vagrant-dns"`, then found [vagrant-dns](https://github.com/BerlinVagrant/vagrant-dns/), which is not a part of my default vagrant installation.
Highlight that vagrant-dns only works on OSX
Merge pull request #37 from pwaller/patch-1

Explain how perfplat.dev is supposed to work