Add examples/ dir, README, and multicluster.py

multicluster.py creates two ONOS "clusters" (1 node by
default, though larger are possible), each of which
is responsible for a separate segment of the data network.

Change-Id: I233c9884b565bd6a28fa1a05e990e86207c88347
diff --git a/tools/dev/mininet/examples/README b/tools/dev/mininet/examples/README
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+This directory contains examples of using onos.py.
+
+You run them directly as Python scripts:
+
+    sudo ./multicluster.py
+
+
+multicluster.py: creates two ONOS controller clusters, each of
+                 which controls a separate segment of the data network.