Add notification controller to docs

docs/components/notification/controller.md

@@ -0,0 +1,16 @@
+# Notification Controller
+
+The Notification Controller is a Kubernetes operator,
+specialized in dispatching events to external systems such as
+Slack, Microsoft Teams, Discord and Rocket chat.
+
+The controller receives events via HTTP and dispatch them to external
+webhooks based on event severity and involved objects.
+
+The controller can be configured with Kubernetes custom resources that
+define how events are processed and where to dispatch them.
+
+Links:
+
+- Source code [fluxcd/notification-controller](https://github.com/fluxcd/notification-controller)
+- Specification [docs](https://github.com/fluxcd/notification-controller/tree/master/docs/spec)