docs: add signals plugin docs

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

plugins/signals-backend/README.md

@@ -6,26 +6,21 @@ Signals plugin allows backend plugins to publish messages to frontend plugins.
 
 ## Getting started
 
-Add Signals router to your backend in `packages/backend/src/plugins/signals.ts`:
+First install the `@backstage/plugin-signals-node` plugin to get the `SignalService` set up.
+
+Next, add Signals router to your backend in `packages/backend/src/plugins/signals.ts`:
 
 ```ts
 import { Router } from 'express';
 import { createRouter } from '@backstage/plugin-signals-backend';
-import { SignalsService } from '@backstage/plugin-signals-node';
 import { PluginEnvironment } from '../types';
 
 export default async function createPlugin(
   env: PluginEnvironment,
 ): Promise<Router> {
-  const service = SignalsService.create({
-    logger: env.logger,
-    identity: env.identity,
-    eventBroker: env.eventBroker,
-  });
-
   return await createRouter({
     logger: env.logger,
-    service,
+    service: env.signalsService,
   });
 }
 ```