Update rpl-udp example readme

examples/rpl-udp/README.md

-A simple RPL network with UDP communication. The DAG root also acts as
-UDP server. All other nodes are client. The clients send a UDP request
-that simply includes a counter as payload. When receiving a request, The
-server sends a reply with the same counter back to the originator.
+A simple RPL network with UDP communication. This is a self-contained example:
+it includes a DAG root (`udp-server.c`) and DAG nodes (`udp-clients.c`).
+This example runs without a border router -- this is a stand-alone RPL network.
+
+The DAG root also acts as UDP server. The DAG nodes are UDP client. The clients
+send a UDP request periodically, that simply includes a counter as payload.
+When receiving a request, The server sends a response with the same counter
+back to the originator.
 
 The simulation files show example networks, for sky motes and for cooja motes.