docs: add info how to add notification processors

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/notifications-backend/README.md

@@ -4,9 +4,6 @@ Welcome to the notifications backend plugin!
 
 ## Getting started
 
-First you have to install `@backstage/plugin-notifications-node` and `@backstage/plugin-signals-node`
-packages.
-
 Add the notifications to your backend:
 
 ```ts
@@ -15,7 +12,68 @@ const backend = createBackend();
 backend.add(import('@backstage/plugin-notifications-backend'));
 ```
 
+For users to be able to see notifications in real-time, you have to install also
+the signals plugin (`@backstage/plugin-signals-node`, `@backstage/plugin-signals-backend`, and
+`@backstage/plugin-signals`).
+
 ## Extending Notifications
 
 The notifications can be extended with `NotificationProcessor`. These processors allow to decorate notifications
 before they are sent or/and send the notifications to external services.
+
+Start off by creating a notification processor:
+
+```ts
+import { Notification } from '@backstage/plugin-notifications-common';
+import { NotificationProcessor } from '@backstage/plugin-notifications-node';
+
+class MyNotificationProcessor implements NotificationProcessor {
+  async decorate(notification: Notification): Promise<Notification> {
+    if (notification.origin === 'plugin-my-plugin') {
+      notification.payload.icon = 'my-icon';
+    }
+    return notification;
+  }
+
+  async send(notification: Notification): Promise<void> {
+    nodemailer.sendEmail({
+      from: 'backstage',
+      to: 'user',
+      subject: notification.payload.title,
+      text: notification.payload.description,
+    });
+  }
+}
+```
+
+Both of the processing functions are optional, and you can implement only one of them.
+
+Add the notification processor to the notification system by:
+
+```ts
+import { notificationsProcessingExtensionPoint } from '@backstage/plugin-notifications-node';
+import { Notification } from '@backstage/plugin-notifications-common';
+
+export const myPlugin = createBackendPlugin({
+  pluginId: 'myPlugin',
+  register(env) {
+    env.registerInit({
+      deps: {
+        notifications: notificationsProcessingExtensionPoint,
+        // ...
+      },
+      async init({ notifications }) {
+        // ...
+        notifications.addProcessor(new MyNotificationProcessor());
+      },
+    });
+  },
+});
+```
+
+## Sending notifications
+
+To be able to send notifications to users, you have to integrate the `@backstage/plugin-notifications-node`
+to your application and plugins.
+See [README](https://github.com/backstage/backstage/blob/master/plugins/plugin-notifications-node/README.md)
+for more information.