move json comma callout to metadata description section

Author: elichad

episodes/03-metadata-descriptor.md

@@ -37,6 +37,22 @@ but they have two different functions.
 The context brings the defined terms into the metadata document,
 while the conformance declares which RO-Crate conventions of using those terms are being followed.
 
+:::::::::::::::::::::::::::::::::::::::: callout
+
+## Adding entities to the JSON array
+
+Because we're adding incrementally to the `@graph` array.	
+It is important to remember the comma `,` between each entity,	
+**except** for the final entity in the JSON array;	
+and likewise for the properties within the JSON object for each entity.	
+This is an artefact of the strict [JSON](https://www.json.org/) file format rules to simplify parsing.	
+The order of the entities within the `@graph` JSON-LD array	
+and the order of the keys within a JSON object is _not significant_.	
+The _graph_ content is given by the `@id` cross-references.	
+
+You will add a comma here between the `ro-crate-metadata.json` entity, and the root data entity.	
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
 ::::::::::::::::::::::::::::::::::::::: callout
 ## RO-Crate versions
 This tutorial is written for RO-Crate 1.2,
@@ -52,5 +68,3 @@ may still be subject to changes and should only be used with caution.
 - RO-Crate specifications are versioned
 - The version of RO-Crate is indicated using the conformsTo property
 ::::::::::::::::::::::::::::::::::::::::::::::::::
-
-