added docs for kafka scaling to kafka source docs (#5850)

Signed-off-by: Calum Murray <[email protected]>

docs/eventing/sources/kafka-source/README.md

@@ -204,6 +204,28 @@ Alternatively, if you are using a GitOps approach, you can add the `consumers` k
 
 ```
 
+### Automatic Scaling with KEDA
+
+Kafka Sources have experimental (Alpha) support for serverless scaling with KEDA, including scale to zero. If you want Knative and KEDA to scale your Kafka source for you,
+you must [install KEDA](https://keda.sh/docs/2.13/deploy/), and then enable the feature flag.
+
+To enable the feature flag, you need to create or modify the `config-kafka-features` configmap in the `knative-eventing` namespace. You can create the file as below:
+
+```yaml
+
+    apiVersion: v1
+    kind: Configmap
+    metadata:
+      name: config-kafka-features
+      namespace: knative-eventing
+    data:
+      controller-autoscaler-keda: "enabled"
+
+```
+
+From there, apply the configmap into your cluster and assuming that KEDA is also installed your Kafka Sources will scale for you! For more information on other values you
+can add to the `config-kafka-features` configmap, [read about the experimental Kafka Broker features](../../brokers/broker-types/kafka-broker/configuring-kafka-features).
+
 
 ### Verify