Add dev notes for fetching description from source plugin (knative#987)

Add TODO note for KafkaSource description text about desired
 mechanism to fetch the description from available source plugin

pkg/kn/commands/source/human_readable_flags.go

@@ -31,7 +31,10 @@ var sourceTypeDescription = map[string]string{
 	"SinkBinding":     "Binding for connecting a PodSpecable to addressable",
 	"PingSource":      "Send periodically ping events to addressable",
 	"ContainerSource": "Generate events by Container image and send to addressable",
-	"KafkaSource":     "Route events from Apache Kafka Server to addressable",
+	// TODO: source plugin could bring the description that kn could look for based on the availability
+	// of the plugin and fetch the description from there, for now we dont have that capability in kn
+	// so we're shipping harcoded short description of the KafkaSource as below
+	"KafkaSource": "Route events from Apache Kafka Server to addressable",
 }
 
 // ListTypesHandlers handles printing human readable table for `kn source list-types`