Document requirement for vagrant-omnibus plugin

[spuder]

If you attempt to start up the vagrant vm, you will get the following error.

sowens-MBP:chef-zookeeper sowen$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'opscode_ubuntu-12.04_provisionerless' could not be found. Attempting to find and install...
    default: Box Provider: virtualbox
    default: Box Version: >= 0
==> default: Adding box 'opscode_ubuntu-12.04_provisionerless' (v0) for provider: virtualbox
    default: Downloading: https://opscode-vm.s3.amazonaws.com/vagrant/opscode_ubuntu-12.04_provisionerless.box
==> default: Successfully added box 'opscode_ubuntu-12.04_provisionerless' (v0) for 'virtualbox'!
There are errors in the configuration of this machine. Please fix
the following errors and try again:

Vagrant:
* Unknown configuration section 'omnibus'.

The solution is to install the plugin

 vagrant plugin install vagrant-omnibus