tools/virtio/ringtest: add usage example to README

Having typical usage example in the README file is more convinient than in
the git history...
Signed-off-by: NMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

tools/virtio/ringtest/README

 Partial implementation of various ring layouts, useful to tune virtio design.
 Uses shared memory heavily.
+
+Typical use:
+
+# sh run-on-all.sh perf stat -r 10 --log-fd 1 -- ./ring