docs: add signals plugin docs

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

plugins/signals-node/README.md

@@ -2,4 +2,69 @@
 
 Welcome to the Node.js library package for the signals plugin!
 
-_This plugin was created through the Backstage CLI_
+Signals plugin allows backend plugins to publish messages to frontend plugins.
+
+## Getting started
+
+Add SignalService to your plugin environment in `packages/backend/src/types.ts`:
+
+```ts
+import { SignalService } from '@backstage/plugin-signals-node';
+
+export type PluginEnvironment = {
+  // ...
+  signalService: SignalService;
+};
+```
+
+Add it also to your `makeCreateEnv` to allow access from the other plugins:
+
+```ts
+import { SignalService } from '@backstage/plugin-signals-node';
+import { DefaultEventBroker } from '@backstage/plugin-events-backend';
+
+function makeCreateEnv(config: Config) {
+  // ...
+
+  const eventBroker = new DefaultEventBroker(root.child({ type: 'plugin' }));
+  const signalService = SignalService.create({
+    logger: root,
+    eventBroker, // EventBroker is optional
+    identity,
+  });
+
+  return (plugin: string): PluginEnvironment => {
+    const logger = root.child({ type: 'plugin', plugin });
+    return {
+      logger,
+      eventBroker,
+      signalService,
+      // ...
+    };
+  };
+}
+```
+
+To allow connections from the frontend, you should also install the `@backstage/plugin-signals-backend`.
+
+## Using the service
+
+Once you have both of the backend plugins installed, you can utilize the signal service by calling the
+`publish` method. This will publish the message to all subscribers in the frontend. To send message to
+all subscribers, you can use `*` as `to` parameter.
+
+```ts
+// Periodic sending example
+setInterval(async () => {
+  // You can use hasSubscribers to check if the message will be sent to anyone
+  // in case you need some heavy processing before that
+  if (signalService.hasSubscribers('plugin:topic')) {
+    await signalService.publish('*', 'plugin:topic', {
+      message: 'hello world',
+    });
+  }
+}, 5000);
+```
+
+To receive this message in the frontend, check the documentation of `@backstage/plugin-signals` and
+`@backstage/plugin-signals-react`.