documentation update for subscription (#450)

* documentation update for subscription

Author: saif-ericsson

wiki/subscriptions.md

@@ -34,7 +34,7 @@ the aggregated object will look like when writing the conditions. The
 requirements inside a subscription should reflect a wanted state of the 
 aggregation, for example: "_Hey Eiffel Intelligence, let me know when my 
 artifact has been published here!_" or "_I want to know when these two products 
-have reached confidence level X_".
+have reached confidence level X_". 
 
 We will go through how to write some example requirements in a subscription
 based on the below aggregated object. What you see in the aggregation is 
@@ -43,6 +43,9 @@ has also added an extra _id key with the value of the start event.
 This _id serves as an index in Mongo DB to speed up searches in the database. 
 The below aggregation is based on the [Artifact rules](../src/main/resources/rules/ArtifactRules-Eiffel-Agen-Version.json).
 
+It should be noted that requirements in subscriptions are written 
+by using [JMESPath](https://jmespath.org/) language. And, here is the [JMESPATH specification](https://jmespath.org/specification.html).
+
 **Aggregation from several Eiffel events:**
 
     {