plugins/app-backend: add installation instructions

plugins/app-backend/README.md

@@ -1,3 +1,32 @@
 # 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.