message: docs: add reference to envoy-filter-example.
  Fixes
diff: @@ -4,5 +4,7 @@ Extending Envoy for custom use cases
  ====================================
  The Envoy architecture makes it fairly easily extensible via both :ref:`network filters
  -<arch_overview_network_filters>` and :ref:`HTTP filters <arch_overview_http_filters>`. Documentation
  -on writing filters is forthcoming.
  +<arch_overview_network_filters>` and :ref:`HTTP filters <arch_overview_http_filters>`.
  +
  +An example of how to add a network filter and structure the repository and build dependencies can
  +be found at `envoy-filter-example <https://github.com/lyft/envoy-filter-example>`_.
  