Adding documentation for rx-java support

docs/src/main/asciidoc/spring-cloud-sleuth.adoc

@@ -407,6 +407,13 @@ In order to pass the tracing information you have to wrap the same logic in the
 include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/hystrix/TraceCommandTests.java[tags=trace_hystrix_command,indent=0]
 ----
 
+=== `RxJavaSchedulersHook` to add support for `RxJava` Schedulers
+
+We're registering a custom https://github.com/ReactiveX/RxJava/wiki/Plugins#rxjavaschedulershook [`RxJavaSchedulersHook`]
+that wraps all `Action0` instances into their Sleuth representative -
+the `TraceAction`. The hook either starts or continues a span depending on the fact whether tracing was already going
+on before the Action was scheduled. To disable the custom RxJavaSchedulersHook set the `spring.sleuth.rxjava.schedulers.hook.enabled` to `false`.
+
 === HTTP integration
 
 Features from this section can be disabled by providing the `spring.sleuth.web.enabled` property with value equal to `false`.