Mention fast datapath port

README.md

@@ -107,9 +107,8 @@ The only difference, apart from the name of the application container,
 is that we tell our weave that it should peer with the weave on
 `$HOST1` (specified as the IP address or hostname, and optional
 `:port`, by which `$HOST2` can reach it). NB: if there is a firewall
-between `$HOST1` and `$HOST2`, you must open the weave port (6783 by
-default; this can be overriden by setting `WEAVE_PORT`) for TCP and
-UDP.
+between `$HOST1` and `$HOST2`, you must permit traffic to the weave
+control port (TCP 6783) and data ports (UDP 6783/6784).
 
 Note that we could instead have told the weave on `$HOST1` to connect to
 `$HOST2`, or told both about each other. Order does not matter here;