message: plugins/app-backend: add installation instructions
diff: # App backend plugin
  This backend plugin can be installed to serve static content of a Backstage app.
  +
  +## Installation
  +
  +Add both this package and your local frontend app package as dependencies to your backend, for example
  +
  +```bash
  +yarn add @backstage/plugin-app-backend example-app
  +```
  +
  +By adding the app package as a dependency we ensure that it is build as part of the backend, and that it can be resolved at runtime.
  +
  +Now add the plugin router to your app, creating it for example like this:
  +
  +```ts
  +const router = await createRouter({
  + logger,
  + appPackageName: 'example-app',
  +});
  +```
  +
  +And registering it like this:
  +
  +```ts
  +createServiceBuilder(module)
  + ...
  + .addRouter('', router);
  +```
  +
  +Be sure to register the app router last, as it serves content for HTML5-mode navigation, i.e. falling back to serving `index.html` for any route that can't be found.
  