explain why sirv is in prod dependencies

README.md

@@ -36,6 +36,17 @@ Navigate to [localhost:5000](http://localhost:5000). You should see your app run
 By default, the server will only respond to requests from localhost. To allow connections from other computers, edit the `sirv` commands in package.json to include the option `--host 0.0.0.0`.
 
 
+## Building and running in production mode
+
+To create an optimised version of the app:
+
+```bash
+npm run build
+```
+
+You can run the newly built app with `npm run start`. This uses [sirv](https://github.com/lukeed/sirv), which is included in your package.json's `dependencies` so that the app will work on platforms that follow the `npm run build`/`npm run start` convention, like [now](https://zeit.co/now).
+
+
 ## Deploying to the web
 
 ### With [now](https://zeit.co/now)