Add flow state machine extension tutorial

docs/source/example-code/src/main/kotlin/net/corda/docs/tutorial/flowstatemachines/TutorialFlowAsyncOperation.kt

@@ -0,0 +1,38 @@
+package net.corda.docs.tutorial.flowstatemachines
+
+import co.paralleluniverse.fibers.Suspendable
+import net.corda.core.concurrent.CordaFuture
+import net.corda.core.flows.FlowLogic
+import net.corda.core.flows.StartableByRPC
+import net.corda.core.internal.FlowAsyncOperation
+import net.corda.core.internal.concurrent.doneFuture
+import net.corda.core.internal.concurrent.openFuture
+import net.corda.core.internal.executeAsync
+
+// DOCSTART SummingOperation
+class SummingOperation(val a: Int, val b: Int) : FlowAsyncOperation<Int> {
+    override fun execute(): CordaFuture<Int> {
+        return doneFuture(a + b)
+    }
+}
+// DOCEND SummingOperation
+
+// DOCSTART SummingOperationThrowing
+class SummingOperationThrowing(val a: Int, val b: Int) : FlowAsyncOperation<Int> {
+    override fun execute(): CordaFuture<Int> {
+        throw IllegalStateException("You shouldn't be calling me")
+    }
+}
+// DOCEND SummingOperationThrowing
+
+// DOCSTART ExampleSummingFlow
+@StartableByRPC
+class ExampleSummingFlow : FlowLogic<Int>() {
+    @Suspendable
+    override fun call(): Int {
+        val answer = executeAsync(SummingOperationThrowing(1, 2))
+        return answer // hopefully 3
+    }
+}
+// DOCEND ExampleSummingFlow
+