Document CloudEvents config (#40)

* Document CloudEvents config

docs/cloud-events.md

@@ -0,0 +1,17 @@
+# CloudEvents
+
+Tekton Pipelines can generate CloudEvents for `TaskRun`, `PipelineRun` and `CustomRun` lifecycle events.
+
+The generation of events for `TaskRun` and `PipelineRun` is automatically enabled only if a URL has been configured for the CloudEvents sink. For example, you can use [Knative Eventing](https://knative.dev/docs/eventing) to set up a listener for such events.
+
+```yaml
+config-defaults:
+  default-cloud-events-sink: "<cloud-events-sink-url>"
+```
+
+If you also want Tekton Pipelines to generate CloudEvents for `CustomRun`, you need to enable the dedicated feature.
+
+```yaml
+feature-flags:
+  send-cloudevents-for-runs: "true"
+```