Add RabbitMQ documentation file

docs/rmq.md

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+# monero-lws RabbitMQ Usage
+Monero-lws uses RabbitMQ to provide JSON notifications of payment_id
+(web)hooks. The feature is optional - by default LWS is _not_ compiled with
+RabbitMQ support. The cmake option -DWITH_RMQ=ON must be specified during the
+configuration stage to enable the feature.
+
+The notification location is specified with `--rmq-address`, the login details
+are specified with `--rmq-credentials` (specified as `user:pass`), the exchange
+is specified with `--rmq-exchange`, and routing name is specified with
+`--rmq-routing`. 
+
+## `json-full-payment_hook`
+Only events of this type are sent to RabbitMQ. They are always "simulcast" with
+the webhook URL. If the webhook URL is `zmq` then no simulcast occurs - the
+event is only sent via ZeroMQ and/or RabbitMQ (if both are configured then it
+sent to both, otherwise it is only sent to the one configured).
+
+Example of the "raw" output to RabbitMQ:
+
+```json
+{
+  "index": 2,
+  "event": {
+    "event": "tx-confirmation",
+    "payment_id": "4f695d197f2a3c54",
+    "token": "single zmq wallet",
+    "confirmations": 1,
+    "event_id": "3894f98f5dd54af5857e4f8a961a4e57",
+    "tx_info": {
+      "id": {
+        "high": 0,
+        "low": 5666768
+      },
+      "block": 2265961,
+      "index": 1,
+      "amount": 3117324236131,
+      "timestamp": 1687301600,
+      "tx_hash": "ef3187775584351cc5109de124b877bcc530fb3fdbf77895329dd447902cc566",
+      "tx_prefix_hash": "064884b8a8f903edcfebab830707ed44b633438b47c95a83320f4438b1b28626",
+      "tx_public": "54dce1a6eebafa2fdedcea5e373ef9de1c3d2737ae9f809e80958d1ba4590d74",
+      "rct_mask": "4cdc4c4e340aacb4741ba20f9b0b859242ecdad2fcc251f71d81123a47db3400",
+      "payment_id": "4f695d197f2a3c54",
+      "unlock_time": 0,
+      "mixin_count": 15,
+      "coinbase": false
+    }
+  }
+}
+```
+> `index` is a counter used to detect dropped messages. It is not useful to
+RabbitMQ but is a carry-over from ZeroMQ (the same serialized message is used
+to send to both).
+
+> The `block` and `id` fields in the above example are NOT present when
+`confirmations == 0`.