added README for CQRS ordered events example

_examples/basic/6-cqrs-ordered-events/README.md

@@ -4,7 +4,24 @@ This application is using [Watermill CQRS](http://watermill.io/docs/cqrs) compon
 
 Detailed documentation for CQRS can be found in Watermill's docs: [http://watermill.io/docs/cqrs#usage](http://watermill.io/docs/cqrs).
 
-This example, uses event groups to maintain order 
+This example, uses event groups to maintain order of events. You can read more about them in the [Watermill documentation](https://watermill.io/docs/cqrs/).
+We are also using Kafka's partitioning keys to increase processing throughput without losing order of events.
+
+
+## What does this application do?
+
+This application manages an email subscription system where users can:
+
+1. Subscribe to receive emails by providing their email address
+2. Update their email address after subscribing
+3. Unsubscribe from the mailing list
+
+The system maintains:
+- A current list of all active subscribers
+- A timeline of all subscription-related activities
+
+If events won't be ordered, and `SubscriberSubscribed` would arrive after `SubscriberUnsubscribed` event,
+the subscriber will be still subscribed.
 
 ## Running