Update docs to Plano (#639)

docs/source/resources/tech_overview/threading_model.rst

@@ -0,0 +1,21 @@
+.. _arch_overview_threading:
+
+Threading Model
+===============
+
+Plano builds on top of Envoy's single process with multiple threads architecture.
+
+A single *primary* thread controls various sporadic coordination tasks while some number of *worker*
+threads perform filtering, and forwarding.
+
+Once a connection is accepted, the connection spends the rest of its lifetime bound to a single worker
+thread. All the functionality around prompt handling from a downstream client is handled in a separate worker thread.
+This allows the majority of Plano to be largely single threaded (embarrassingly parallel) with a small amount
+of more complex code handling coordination between the worker threads.
+
+Generally, Plano is written to be 100% non-blocking.
+
+.. tip::
+
+   For most workloads we recommend configuring the number of worker threads to be equal to the number of
+   hardware threads on the machine.