contrib: add first draft of RPC docs

contributing/rpc/README.md

@@ -1,6 +1,50 @@
 # RPC
 
-- net/rpc - (in the stdlib)
+This section is a work in progress.
-- [Streaming](./streaming)
-- routing of "RPC" requests
 
+The RPC subsystem is exclusicely in Server Agents. It is comprised of two main components:
+
+1. the "RPC Server" (for lack of a better term) handles multiplexing of many different
+   requests on a single TCP port.
+2. RPC endpoints handle RPC requests and return responses.
+
+The RPC subsystems handles requests from:
+
+1. Client Agents in the local DC
+2. (if the server is a leader) other Server Agents in the local DC
+3. Server Agents in other Datacenters
+4. in-process requests from other components running in the same process (ex: the HTTP API
+   or DNS interface).
+
+## Routing
+
+The "RPC Server" accepts requests to the [server port] and routes the requests based on
+configuration of the Server and the the first byte in the request. The diagram below shows
+all the possible routing flows.
+
+[server port]: https://www.consul.io/docs/agent/options#server_rpc_port
+
+![RPC Routing](./routing.svg)
+
+<sup>[source](./routing.mmd)</sup>
+
+The main entrypoint to RPC routing is `handleConn` in [agent/consul/rpc.go].
+
+[agent/consul/rpc.go]: https://github.com/hashicorp/consul/blob/main/agent/consul/rpc.go
+
+
+## RPC Endpoints
+
+This section is a work in progress, it will eventually cover topics like:
+
+- net/rpc - (in the stdlib)
+- new grpc endpoints
+- [Streaming](./streaming)
+
+
+## RPC connections and load balancing
+
+This section is a work in progress, it will eventually cover topics like:
+
+- agent/router
+- agent/pool