Initial commit of Sphinx docs

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Signed-off-by: John DeNisco <jdenisco@cisco.com>

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+.. _extensible:
+
+=============================
+Extensible and Modular Design
+=============================
+
+* Pluggable, easy to understand & extend
+* Mature graph node architecture
+* Full control to reorganize the pipeline
+* Fast, plugins are equal citizens
+
+**Modular, Flexible, and Extensible**
+
+The FD.io VPP packet processing pipeline is decomposed into a ‘packet processing
+graph’.  This modular approach means that anyone can ‘plugin’ new graph
+nodes. This makes VPP easily exensible and means that plugins can be
+customized for specific purposes. VPP is also configurable through it's
+Low-Level API.
+
+.. figure:: /_images/VPP_custom_application_packet_processing_graph.280.jpg
+   :alt: Extensible, modular graph node architecture?
+   
+   Extensible and modular graph node architecture. 
+
+At runtime, the FD.io VPP platform assembles a vector of packets from RX rings,
+typically up to 256 packets in a single vector. The packet processing graph is
+then applied, node by node (including plugins) to the entire packet vector. The
+received packets typically traverse the packet processing graph nodes in the
+vector, when the network processing represented by each graph node is applied to
+each packet in turn.  Graph nodes are small and modular, and loosely
+coupled. This makes it easy to introduce new graph nodes and rewire existing
+graph nodes.
+
+Plugins are `shared libraries <https://en.wikipedia.org/wiki/Library_(computing)>`_ 
+and are loaded at runtime by VPP. VPP find plugins by searching the plugin path 
+for libraries, and then dynamically loads each one in turn on startup. 
+A plugin can introduce new graph nodes or rearrange the packet processing graph. 
+You can build a plugin completely independently of the FD.io VPP source tree,
+which means you can treat it as an independent component.