dump: add README and manpage

refs #2855

Change-Id: I59379349e3579b267956933aeb603aa8d747aa53
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+# ndndump
+
+**ndndump** is a traffic analysis tool that captures Interest and Data packets on the wire,
+and displays brief information about captured packets.
+
+Usage example:
+
+1. start NFD on local machine
+2. create an IPv4 UDP tunnel to a remote machine
+3. cause some traffic going on the tunnel
+4. execute `sudo ndndump`
+
+For more information, consult the manpage.