Added docs for launching a CT with custom CTR

resolves #3377

Updated based on code review

Author: cppwfs

spring-cloud-dataflow-docs/src/main/asciidoc/tasks.adoc

@@ -619,6 +619,22 @@ dataflow:>task launch my-composed-task --arguments "--increment-instance-enabled
 Launched task 'my-composed-task'
 ----
 
+===== Launching a Composed Task using Custom Composed Task Runner
+
+In some cases a user will need to launch a composed task using a custom version of a Composed Task Runner other than default application that is shipped out-of-the-box.
+To do this, a user will need to register the custom version of the Composed Task Runner and then specify the --composedTaskRunnerName property pointing to the custom application at task launch as shown below:
+[source,bash,options="nowrap"]
+----
+dataflow:>app register --name best-ctr --type task --uri maven://the.best.ctr.composed-task-runner:1.0.0.RELEASE
+
+dataflow:>task create mycomposedtask --definition "te:timestamp &&  tr:timestamp"
+Created new task 'mycomposedtask'
+
+dataflow:>task launch --name mycomposedtask --composedTaskRunnerName best-ctr
+----
+
+NOTE: The app specified by the  `composedTaskRunnerName` needs to be a task registered in the Application Registry.
+
 ===== Exit Statuses
 
 The following list shows how the Exit Status is set for each step (task) contained in the composed task following each step execution: