Add an example of OS::Mistral::ExternalResource

I couldn't find a simple example of that resource, so here's a real
simplified one creating a workflow in the same template.

Change-Id: I9de98df8a629227ba83e50382ccba660621fcbd5

hot/mistral_external.yaml

@@ -0,0 +1,20 @@
+heat_template_version: rocky
+
+resources:
+
+  workflow:
+    type: OS::Mistral::Workflow
+    properties:
+      type: direct
+      tasks:
+        - name: hello
+          action: std.echo output='Good morning!'
+          publish:
+            result: 'Bye'
+
+  external_resource:
+    type: OS::Mistral::ExternalResource
+    properties:
+      actions:
+         CREATE:
+           workflow: {get_resource: workflow}