Add guide from issue #206 (#228)

Add guide from issue #206

Closes #206

Co-authored-by:  Matthew Phillips <[email protected]>

docs/guides.md

@@ -9,4 +9,5 @@ These guides are meant to help you along your journey learning Finite State Mach
 
 * __[Composition](./guides/composition.html)__: How to use functional composition when building state machines, to maximize code reuse where possible.
 * __[Nested States](./guides/nested-states.html)__: Create machines with nested states by [invoking](./api/invoke.html) other machines.
-* __[Comparison with XState](./guides/comparison-with-xstate.html)__: Differences and tradeoffs between Robot and [XState](https://xstate.js.org).
+* __[Awaiting asynchronous completion](./guides/awaiting-asynchronous-completion.html)__: How to await a machine to enter a `final` state.
+* __[Comparison with XState](./guides/comparison-with-xstate.html)__: Differences and tradeoffs between Robot and [XState](https://xstate.js.org).

docs/guides/awaiting-asynchronous-execution.md

@@ -0,0 +1,30 @@
+---
+layout: page.njk
+title: Awaiting asynchronous execution
+tags: guide
+permalink: guides/awaiting-asynchronous-execution.md
+---
+
+In a scenario where it's necessary to `await` the machine to enter a `final`
+state (a state which has no transitions), the `onChange` callback (the second argument to `interpret`) can be used
+to resolve a promise. The promise can be externally awaited.
+
+```js
+let resolve;
+let promise = new Promise(_resolve => {
+  resolve = _resolve;
+})
+
+service = interpret(machine, () => {
+  if(machine.state.value.final) {
+    resolve();
+  }
+});
+
+await promise;
+// All done!
+```
+
+This is particularly useful for state machines which consist entirely
+of `immediate` and `invoke` states: these model execution flows which do
+not depend on external events for their execution and state transition.