GH-590 Add initial RSocket documentation

docs/src/main/asciidoc/index.adoc

@@ -10,6 +10,7 @@ The reference documentation consists of the following sections:
 [horizontal]
 <<spring-cloud-function.adoc#,Reference Guide>> :: Spring Cloud Function Reference
 https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-samples/function-sample-cloudevent[Cloud Events] :: Cloud Events
+https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-rsocket[RSocket] :: RSocket
 <<aws.adoc#,AWS Adapter>> :: AWS Adapter Reference
 <<azure.adoc#, Azure Adapter>> :: Azure Adapter Reference
 <<gcp.adoc#, GCP Adapter>> :: GCP Adapter Reference

spring-cloud-function-rsocket/README.adoc

@@ -0,0 +1,59 @@
+### Introduction 
+
+Spring Cloud Function allows you to invoke function via [RSocket](https://rsocket.io/). While you can read more about RSocket and it’s java 
+implementation [here](https://github.com/rsocket/rsocket-java), this section will describe the parts relevant to Spring Cloud Function integration.
+
+### Programming model
+From the user perspective bringing RSocket does not change the implementation of functions or any of its features, such as type conversion, 
+composition, POJO functions etc.
+And while RSocket allows first class reactive interaction over the network supporting important reactive features such as back pressure, 
+users of Spring Cloud Function still have freedom to implement their business logic using reactive or imperative functions delegating any 
+adjustment needed to apply proper invocation model to the framework.
+
+To use RSocket integration all you need is to add `spring-cloud-function-rsocket` dependency to your classpath
+```
+<dependency>
+            <groupId>org.springframework.cloud</groupId>
+            <artifactId>spring-cloud-function-rsocket</artifactId>
+            <version>3.1.0</version>
+ </dependency>
+```
+
+To interact with functions via RSocket we rely on Spring Boot support for RSocket and `RSocketRequester.Builder` API.
+The code below shows the key parts and you can get more details on various interaction models 
+from [this test case](https://github.com/spring-cloud/spring-cloud-function/blob/master/spring-cloud-function-rsocket/src/test/java/org/springframework/cloud/function/rsocket/RSocketAutoConfigurationTests.java).
+
+
+```
+@Bean
+public Function<String, String> uppercase() {
+	return v -> v.toUpperCase();
+}
+
+. . .
+
+RSocketRequester.Builder rsocketRequesterBuilder =
+				applicationContext.getBean(RSocketRequester.Builder.class);
+
+rsocketRequesterBuilder.tcp("localhost", port)
+	.route(“uppercase")
+	.data("\"hello\"")
+	.retrieveMono(String.class)
+	.subscribe(System.out::println);
+```
+
+Once connected to RSocket we use `route` operation to specify which function we want to invoke providing the actual 
+payload via `data` operation. Then we use one of the `retrieve` operations that best suits our desired interaction 
+(RSocket supports multiple interaction models such as fire-and-forget, request-reply etc.)
+
+If you want to provide additional information that you would normally communicate via Message headers, you can use `metadata` operation for that.
+```
+rsocketRequesterBuilder.tcp("localhost", port)
+	.route(“uppercase”)
+	.metadata("{\”header_key\":\”header-value\"}", MimeTypeUtils.APPLICATION_JSON)		
+	.data("\"hello\"")
+	.retrieveMono(String.class)
+	.subscribe(System.out::println);
+```
+
+You can also look at one of the [RSocket samples](https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-samples/function-sample-cloudevent-rsocket) that is also introduces you to Cloud Events 

spring-cloud-function-rsocket/src/test/java/org/springframework/cloud/function/rsocket/RSocketAutoConfigurationTests.java

@@ -469,9 +469,7 @@ public class RSocketAutoConfigurationTests {
 
 		@Bean
 		public Function<String, String> uppercase() {
-			return v -> {
+			return v -> v.toUpperCase();
-				return v.toUpperCase();
-			};
 		}
 
 		@Bean