add README

Author: dominikh

README

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+This is a reimplementation of the netstat-nat[1] tool, written
+entirely in Go. It uses the same command line flags and almost the
+same output format so it can be used as a drop-in replacement in most
+cases.
+
+Differences
+-----------
+
+- The original version limits the printed hostnames to fixed width. We
+  do not.
+
+- The -x flag is a NOOP because we do not limit the length of
+  hostnames.
+
+- The -N flag is not yet supported.
+
+Links
+-----
+
+[1]: http://tweegy.nl/projects/netstat-nat/