Update README for Vagrant

This repo now has a Vagrantfile which means that you don't need to download
and import the meta-appliance separately. Update the README to reflect this
simpler method, with a couple of caveats.

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Remove shared folders from Vagrantfile - Boxgrinder complains as follows if you try to run it from the shared folder:

VMBuilder.exception.VMBuilderException: Process (['/usr/sbin/debootstrap', '--arch=amd64', 'precise', 'build/appliances/x86_64/ubuntu/precise/ubuntu-precise/1.0/ubuntu-plugin/tmp/tmpqgZ2wY', 'http://arc
hive.ubuntu.com/ubuntu']) returned 1. stdout: E: Cannot install into target '/home/vagrant/boxgrinder-appliances/build/appliances/x86_64/ubuntu/precise/ubuntu-precise/1.0/ubuntu-plugin/tmp/tmpqgZ2wY' mo
unted with noexec or nodev
, stderr: mknod: `/home/vagrant/boxgrinder-appliances/build/appliances/x86_64/ubuntu/precise/ubuntu-precise/1.0/ubuntu-plugin/tmp/tmpqgZ2wY/test-dev-null': Operation not permitted

So remove them, in order to save confusion with `/root/boxgrinder-appliances`.

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/cc @samjsharpe @annashipman @nickstenning