Add install instructions for older nevironments

Signed-off-by: Marek Libra <marek.libra@gmail.com>

plugins/signals-backend/README.md

@@ -6,41 +6,4 @@ Signals plugin allows backend plugins to publish messages to frontend plugins.
 
 ## Getting started
 
-First install the `@backstage/plugin-signals-node` plugin to get the `SignalsService` 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 { PluginEnvironment } from '../types';
-
-export default async function createPlugin(
-  env: PluginEnvironment,
-): Promise<Router> {
-  return await createRouter({
-    logger: env.logger,
-    eventBroker: env.eventBroker,
-    identity: env.identity,
-    discovery: env.discovery,
-  });
-}
-```
-
-Now add the signals to `packages/backend/src/index.ts`:
-
-```ts
-// ...
-import signals from './plugins/signals';
-
-async function main() {
-  // ...
-  const signalsEnv = useHotMemoize(module, () => createEnv('signals'));
-
-  const apiRouter = Router();
-  // ...
-  apiRouter.use('/signals', await signals(signalsEnv));
-  apiRouter.use(notFoundHandler());
-  // ...
-}
-```
+To install this signals backend plugin, please refer the [Getting Started](https://backstage.io/docs/notifications) Backstage Notifications and Signals documentation section.