docs: add README to @angular/pwa.

This mostly just copies the "Getting started with service workers" guide cited to explain what the schematic itself does and defering to that doc for more info on PWA's and how to actually build one.

(cherry picked from commit d09da29)

packages/angular/pwa/README.md

@@ -0,0 +1,22 @@
+# `@angular/pwa`
+
+This is a [schematic](https://angular.io/guide/schematics) for adding
+[Progress Web App](https://web.dev/progressive-web-apps/) support to an Angular app. Run the
+schematic with the [Angular CLI](https://angular.io/cli):
+
+```shell
+ng add @angular/pwa
+```
+
+This makes a few changes to your project:
+
+1. Adds [`@angular/service-worker`](https://npmjs.com/@angular/service-worker) as a dependency.
+1. Enables service worker builds in the Angular CLI.
+1. Imports and registers the service worker in the app module.
+1. Adds a [web app manifest](https://developer.mozilla.org/en-US/docs/Web/Manifest).
+1. Updates the `index.html` file to link to the manifest and set theme colors.
+1. Adds required icons for the manifest.
+1. Creates a config file `ngsw-config.json`, specifying caching behaviors and other settings.
+
+See [Getting started with service workers](https://angular.io/guide/service-worker-getting-started)
+for more information.