add doc

doc/bookdown.json

@@ -6,7 +6,8 @@
     {"event_store": "event_store.md"},
     {"projection_manager": "projection_manager.md"},
     {"event_store_bus_bridge": "event_store_bus_bridge.md"},
-    {"configuration_reference": "configuration_reference.md"}
+    {"configuration_reference": "configuration_reference.md"},
+    {"messenger": "messenger.md"}
   ],
   "target": "./html",
   "tocDepth": 2,

doc/messenger.md

@@ -0,0 +1,24 @@
+# Messenger integration
+
+This bundle provides a middleware for the
+`symfony/messenger` component (from version `4.3`) which handles
+starting/committing/rolling back a transaction when sending a command
+to the bus.
+
+Here is an example configuration on how to use it:
+```yaml
+# app/config/messenger.yaml
+
+framework:
+    messenger:
+        buses:
+            command.bus:
+                middleware:
+                    - my_eventstore_transaction_middleware
+
+services:
+    my_eventstore_transaction_middleware:
+        class: Prooph\Bundle\EventStore\Messenger\EventStoreTransactionMiddleware
+        arguments:
+            - '@my_transactional_event_store'
+```