Prioritize user documentation over README (notifications, signals)

Selected content of notifications and signals README files has been
moved upwards to the user documentation.

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

plugins/notifications/README.md

@@ -2,13 +2,17 @@
 
 Welcome to the notifications plugin!
 
-_This plugin was created through the Backstage CLI_
-
 ## Getting started
 
 First, install the `@backstage/plugin-notifications-backend` and `@backstage/plugin-notifications-node` packages.
 See the documentation for installation instructions.
 
+Then add this frontend package:
+
+```bash
+yarn workspace app add @backstage/notifications
+```
+
 To add the notifications main menu, add the following to your `packages/app/src/components/Root/Root.tsx`:
 
 ```tsx