Update README

README.md

@@ -46,6 +46,26 @@ Desired properties:
   design not only the proper RPC API, but a mechanism that can
   tolerate (temporary) failure at that stage.
 
+## Usage
+
+The service has two main components: the *engine* (server), and the
+*worker*. You should, in most cases, co-locate a shard's worker with
+that shard's engine.
+
+Both components can be started using the *enq* command-line tool, with
+either the *server* or *worker* command. Check their specific *--help*
+outputs for further configuration options.
+
+The components of the service talk GRPC between themselves, and they
+can authenticate each other via mutual TLS, or via a shared
+authentication token (or both).
+
+The worker will handle tasks by running external processes found in
+the */etc/enq/worker.d/* directory (or wherever you set the *--dir*
+option to), named after task methods. Such processes are passed task
+arguments as a JSON-encoded list of objects on standard input, and are
+expected to return a JSON-encoded result on standard output.
+
 ## Internals
 
 ### Data model