feat: integrate notifications with signals

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

plugins/notifications-backend/README.md

@@ -4,7 +4,8 @@ Welcome to the notifications backend plugin!
 
 ## Getting started
 
-First you have to install the `@backstage/plugin-notifications-node` package.
+First you have to install `@backstage/plugin-notifications-node` and `@backstage/plugin-signals-node` 
+packages.
 
 Then create a new file to `packages/backend/src/plugins/notifications.ts`:
 
@@ -22,6 +23,7 @@ export default async function createPlugin(
     tokenManager: env.tokenManager,
     database: env.database,
     discovery: env.discovery,
+    signalService: env.signalService
   });
 }
 ```