Document connection origin for relay listener

Make it explicit that the connection is established by an outbound
connection from the relay listener inside the Trust network boundary.

Author: jabley

docs/relay-listener/README.md

@@ -10,6 +10,14 @@ The listener processes worklist actions sent from Manage/Django and creates work
 ┌─────────────────────┐                           ┌──────────────────────┐
 │   Django (Manage)   │                           │  Gateway (Behind FW) │
 └─────────────────────┘                           └──────────────────────┘
+         │                                                          │
+         │                                                          │
+         │                                             (0) Connect  │
+         │                       <────────────────────────────────  │
+         │          Establish a connection. Once this succeeds, we  │
+         │          will have a full-duplex, bi-directional comms   │
+         │          channel                                         │
+         │                                                          │
          │                                                          │
          │ (1) Send Worklist Actions                                │
          │ ────────────────────────────────>                        │