docs: add long description to app-shell subcommand

packages/schematics/angular/app-shell/app-shell-long.md

@@ -0,0 +1,47 @@
+An app shell lets Universal render a portion of your application via a route at build time.
+This gives users a meaningful first paint of your application that appears quickly
+because the browser can simply render the HTML without the need to initialize any JavaScript.
+
+Use this command with a routing app that is accompanied by a Universal server-side app.
+
+To create an app shell, use the following command.
+
+<code-example format="." language="bash">
+ ng generate app-shell --client-project my-app --universal-project server-app
+</code-example>
+
+* `my-app` is the name of your client application
+* `server-app` is the name of the Universal (server) application
+
+The command adds two new architect build targets to your `angular.json` configuration file (along with a few other changes).
+
+<code-example  format="." language="none" linenums="false">
+"server": {
+  "builder": "@angular-devkit/build-angular:server",
+  "options": {
+    "outputPath": "dist/my-app-server",
+    "main": "src/main.server.ts",
+    "tsConfig": "src/tsconfig.server.json"
+  }
+},
+"app-shell": {
+  "builder": "@angular-devkit/build-angular:app-shell",
+  "options": {
+    "browserTarget": "my-app:build",
+    "serverTarget": "my-app:server",
+    "route": "shell"
+  }
+}
+</code-example>
+
+To verify the that the app has been built with the default shell content:
+
+1. Run the app-shell target.
+
+   <code-example format="." language="bash">
+       ng run my-app:app-shell
+    </code-example>
+
+1. Open `dist/app-shell/index.html` in your browser.
+
+The default text "app-shell works!" verifies that the app-shell route was rendered as part of the output.

packages/schematics/angular/app-shell/schema.json

@@ -4,7 +4,7 @@
   "title": "Angular AppShell Options Schema",
   "type": "object",
   "description": "Generates an app shell for running a server-side version of an app.",
-  "long-description": "",
+  "long-description": "./app-shell-long.md",
   "properties": {
     "clientProject": {
       "type": "string",