Merge pull request #1465 from itowlson/quick-start-watch

Mention `spin watch` in quickstart

Author: itowlson

content/spin/v3/quickstart.md

@@ -867,6 +867,8 @@ Available Routes:
   hello-typescript: http://127.0.0.1:3000 (wildcard)
 ```
 
+> You can also run a Spin application using `spin watch`. This automatically rebuilds code and reloads content whenever they change. [Learn more.](./running-apps.md#monitoring-applications-for-changes)
+
 Spin instantiates all components from the application manifest, and
 creates the router configuration for the HTTP trigger according to the routes in the manifest. The
 component can now be invoked by making requests to `http://localhost:3000/`