add NFS to prerequisites (for linux hosts)

[davidemoro]

Add NFS to requirements for linux hosts in readme file/docs (from vagrant docs: "the host machine must have nfsd installed, the NFS server daemon. This comes pre-installed on Mac OS X, and is typically a simple package install on Linux.")

Otherwise vagrant up will fail:

It appears your machine doesn't support NFS, or there is not an
adapter to enable NFS on this machine for Vagrant. Please verify
that `nfsd` is installed on your machine, and try again. If you're
on Windows, NFS isn't supported. If the problem persists, please
contact Vagrant support.

[Taytay]

Done! Thanks!