docs: added an explanation about why service worker is included only in production mode (#3050) [ci skip]

closes #2278

Author: NataliaTepluhina

packages/@vue/cli-plugin-pwa/README.md

@@ -2,6 +2,12 @@
 
 > pwa plugin for vue-cli
 
+The service worker added with this plugin is only enabled in the production environment (e.g. only if you run `npm run build` or `yarn build`). Enabling service worker in a development mode is not a recommended practice, because it can lead to the situation when previously cached assets are used and the latest local changes are not included.
+
+Instead, in the development mode the `noopServiceWorker.js` is included. This service worker file is effectively a 'no-op' that will reset any previous service worker registered for the same host:port combination.
+
+If you need to test a service worker locally, build the application and run a simple HTTP-server from your build directory. It's recommended to use a browser incognito window to avoid complications with your browser cache.
+
 ## Configuration
 
 Configuration is handled via the `pwa` property of either the `vue.config.js`
@@ -54,7 +60,7 @@ file, or the `"vue"` field in `package.json`.
   - Default: `'default'`
 
 - **pwa.assetsVersion**
-  
+
   - Default: `''`
 
     This option is used if you need to add a version to your icons and manifest, against browser’s cache. This will append `?v=<pwa.assetsVersion>` to the URLs of the icons and manifest.