Sketching more topology related stuff.
Added overview.html for Java API docs.
diff --git a/src/main/javadoc/overview.html b/src/main/javadoc/overview.html
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+++ b/src/main/javadoc/overview.html
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+<html>
+<body>
+<img src="doc-files/onos-tiers.png" alt="ONOS architecture tiers"><br>
+ONOS architecture is strictly segmented into protocol-agnostic system core and
+protocol-aware providers tiers.
+
+<em>More information to come later...</em>
+
+</body>
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