docs: ServiceWorkerPlugin docs

Author: zetlen

README.md

@@ -20,4 +20,5 @@ a Peregrine app to a Magento backend and a
   Divides static assets into bundled "chunks" based on components registered
   with the Magento PWA `RootComponent` interface
 * [`PWADevServer`](docs/PWADevServer.md) -- Configures your system settings and
-  Webpack configuration for secure, PWA-enabled local development
+* [`ServiceWorkerPlugin`](docs/ServiceWorkerPlugin.md) -- Creates
+  a ServiceWorker with different settings based on dev scenarios

docs/ServiceWorkerPlugin.md

@@ -0,0 +1,70 @@
+## ServiceWorkerPlugin
+
+Webpack plugin for configuring a ServiceWorker for different PWA development
+scenarios.
+
+### Usage
+
+In `webpack.config.js`:
+
+```js
+const path = require('path');
+const buildpack = require('@magento/pwa-buildpack');
+const ServiceWorkerPlugin = buildpack.Webpack.ServiceWorkerPlugin;
+
+module.exports = async env => {
+    const config = {
+        /* webpack config, i.e. entry, output, etc. */
+        plugins: [
+            /* other plugins */
+            new ServiceWorkerPlugin({
+                env: {
+                    mode: 'development'
+                },
+
+                paths: {
+                    output: path.resolve(__dirname, 'web/js'),
+                    assets: path.resolve(__dirname, 'web')
+                },
+                enableServiceWorkerDebugging: true,
+                serviceWorkerFileName: 'sw.js',
+                runtimeCacheAssetPath: 'https://cdn.url'
+            })
+        ]
+    };
+
+    return config;
+
+};
+```
+
+### Purpose
+
+This plugin is a wrapper around the [Google Workbox Webpack Plugin](https://developers.google.com/web/tools/workbox/guides/generate-service-worker/),
+which generates a caching ServiceWorker based on assets emitted by Webpack.
+
+In development, ServiceWorkers can cache assets in a way that interferes with
+real-time editing and changes. This plugin takes configuration that can switch
+between "normal development mode", where ServiceWorker is disabled, to "service
+worker debugging mode", where ServiceWorker is enabled and hot-reloading.
+
+### API
+
+
+#### `new ServiceWorkerPlugin(options: PluginOptions): Plugin`
+
+#### `options`
+
+ - `env: object`: **Required.** An object representing the current environment.
+     - `mode: string`: **Required**. Must be `'development'` or `'production'`.
+ - `paths: object`: **Required.** Local absolute paths to theme folders.
+     - `assets: string`: Directory for public static assets.
+ - `enableServiceWorkerDebugging: boolean`: When `true`, the ServiceWorker will
+   be active at document root (irrespective of publicPath) and hot reloading.
+   When `false`, ServiceWorker will be disabled so that asset hot reloading is
+   not interrupted by cache.
+ - `serviceWorkerFileName: string`: **Required.** The name of the ServiceWorker
+   file this theme creates, e.g. `'sw.js'`.
+ - `runtimeCacheAssetPath: string`: **Required.** A path, or remote URL,
+   representing the root path of assets which the ServiceWorker should cache at
+   runtime as they are requested.