Revise documentation of UDP performance issues based on recent
experiments.

Fixes Issue #55 (at least to the extent that it's not really an
iperf3 issue).

(cherry picked from commit e4e22a58a7094e1372ecf677ea1477cbcf63a337)
Signed-off-by: Bruce A. Mah <bmah@es.net>

Conflicts:
	README.md