ovs-vtep: Clarifications on the README.

The README was not clear on whether the steps are for attaching physical
or virtual machines to the emulated VTEP. This adds more detail to the
README so that readers will know what to do if they want to attach physical
machines or otherwise.

Signed-off-by: Mark Maglana <mmaglana@gmail.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>