Add a basic example to Spring Integration with Reactive Streams

src/reference/asciidoc/reactive-streams.adoc

@@ -167,6 +167,44 @@ A reactive outbound channel adapter implementation is about initiation (or conti
 An inbound payload could be a reactive type per se or as an event of the whole integration flow which is a part of reactive stream on top.
 A returned reactive type can be subscribed immediately if we are in one-way, fire-and-forget scenario, or it is propagated downstream (request-reply scenarios) for further integration flow or an explicit subscription in the target business logic, but still downstream preserving reactive streams semantics.
 
+Example for a reactive inbound channel adapter:
+```java
+public class ReactiveSourceMessageProducer extends MessageProducerSupport {
+
+    private final ReactiveSource myReactiveSource;
+
+    public ReactiveSourceMessageProducer(ReactiveSource myReactiveSource) {
+        Assert.notNull(myReactiveSource, "'myReactiveSource' must not be null");
+        this.myReactiveSource = myReactiveSource;
+    }
+
+    @Override
+    protected void doStart() {
+        Flux < Message << ? >> messageFlux =
+            this.myReactiveSource
+            .map(event - >
+                MessageBuilder
+                .withPayload(event.getBody())
+                .setHeader(MyReactiveHeaders.SOURCE_NAME, event.getSourceName())
+                .build());
+
+        subscribeToPublisher(messageFlux);
+    }
+
+}
+```
+And now we will be able to use it:
+```java
+public class MainFlow {
+
+  @Bean
+  public IntegrationFlow buildFlow() {
+     return IntegrationFlows.from(new ReactiveSourceMessageProducer(new ReactiveSource()))
+        .channel(outputChannel)
+        .get();
+  }
+}
+```
 Currently Spring Integration provides channel adapter (or gateway) implementations for <<./webflux.adoc#webflux,WebFlux>>, <<./rsocket.adoc#rsocket,RSocket>>, <<./mongodb.adoc#mongodb,MongoDb>> and <<./r2dbc.adoc#r2dbc,R2DBC>>.
 The <<./redis.adoc#redis-stream-outbound,Redis Stream Channel Adapters>> are also reactive and uses `ReactiveStreamOperations` from Spring Data.
 Also an https://github.com/spring-projects/spring-integration-extensions/tree/main/spring-integration-cassandra[Apache Cassandra Extension] provides a `MessageHandler` implementation for the Cassandra reactive driver.