Add documentation for event subscription

docs/cache-event-task.md

 # Task Service - Cache Event Task Documentation
 
+### Subscribe for Cache events
+
+Current implementation uses NATS as a messaging system for events.
+There are two environment variables that need to be set for subscribing for cache events.
+
+```shell
+NATS_ADDR="example.com:4222"
+NATS_SUBJECT="subject"
+```
+
 ### Event Task definition
 
 In order to create a Task upon receiving a Cache event an `event task template` **must**