Added the reference to the article

Author: Vlad Balin

docs/chapters/record.md

@@ -372,6 +372,8 @@ All nested records and collections are *aggregated* by default and behave as int
 - They are validated as part of the record.
 - They are serialized as nested JSON.
 
+The nature of aggregation relationship in OO is explained in this [article](https://medium.com/@gaperton/nestedtypes-2-0-meet-an-aggregation-and-the-rest-of-oo-animals-a9fca7c36ecf).
+
 ### `attrDef` : RecordOrCollection
 
 Aggregated record or collection. Represented as nested object or array in record's JSON. Aggregated members are owned by the record and treated as its _integral part_ (recursively created, cloned, serialized, validated, and disposed).

docs/index.html

@@ -507,6 +507,7 @@ <h2 id="nested-records-and-collections">Nested records and collections</h2>
 <li>They are validated as part of the record.</li>
 <li>They are serialized as nested JSON.</li>
 </ul>
+<p>The nature of aggregation relationship in OO is explained in this <a href="https://medium.com/@gaperton/nestedtypes-2-0-meet-an-aggregation-and-the-rest-of-oo-animals-a9fca7c36ecf">article</a>.</p>
 <h3 id="-attrdef-recordorcollection"><code>attrDef</code> : RecordOrCollection</h3>
 <p>Aggregated record or collection. Represented as nested object or array in record&#39;s JSON. Aggregated members are owned by the record and treated as its <em>integral part</em> (recursively created, cloned, serialized, validated, and disposed).
 One object can have single owner. The record with its aggregated attributes forms an <em>aggregation tree</em>.</p>