README: update list of validated OS's

Add Fedora 20 to list of operating systems on which Intel DPDK vSwitch
has been validated.

Change-Id: I60867c9e79778252d80173e5bc51269e85ab89e2
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Reviewed-By: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-By: Kevin Traynor <kevin.traynor@intel.com>