doc(sse) Server-sent event feature

See nestjs/nest#4842

Author: soyuka

content/techniques/server-sent-events.md

@@ -0,0 +1,44 @@
+### Server-Sent Events
+
+You can enable Server-Sent events on a route with Nest. This can be used to push real-time updates to your client using HTTP. More informations about this specification can be found [on the whatwg website](https://html.spec.whatwg.org/multipage/server-sent-events.html).
+
+#### Usage
+
+In this example, we set a route named `/sse` that will allow you to propagate real time updates. These events can be listened to using the Javascript [EventSource API](https://developer.mozilla.org/en-US/docs/Web/API/EventSource). On the `/` route, for the sake of the example, we're logging the received data in the browser's console.
+
+Note the `@Sse` annotation that hooks the Server-Sent events mechanism on the choosed path:
+
+```
+@Controller()
+export class AppController {
+  @Get()
+  getHello(): string {
+    return `
+    <script type="text/javascript">
+      const ee = new EventSource('/sse')
+      ee.onmessage = ({data}) => {
+        console.log('New message', JSON.parse(data))
+      }
+    </script>
+    `;
+  }
+
+  @Sse('/sse')
+  sse(): Observable<MessageEvent> {
+    return interval(1000).pipe(map((_) => ({ data: { hello: 'world' } })));
+  }
+}
+```
+
+The events are sent via an Observable emitting a `MessageEvent`. The MessageEvent should respect the following interface to match the specification:
+
+```
+export interface MessageEvent {
+  data: string | object;
+  id?: string;
+  type?: string;
+  retry?: number;
+}
+```
+
+You can find a sample of a working example on [Nest repository](https://github.com/nestjs/nest/tree/master/sample/28-sse).