docs(entity): detail usage of class instances with entity state adapter (#1561)

Author: jorroll

projects/ngrx.io/content/guide/entity/index.md

@@ -11,3 +11,13 @@ Entity provides an API to manipulate and query entity collections.
 ## Installation 
 
 Detailed installation instructions can be found on the [Installation](guide/entity/install) page.
+
+## Entity and class instances
+
+Entity promotes the use of plain JavaScript objects when managing collections. *ES6 class instances will be transformed into plain JavaScript objects when entities are managed in a collection*. This provides you with some assurances when managing these entities:
+
+1. Guarantee that the data structures contained in state don't themselves contain logic, reducing the chance that they'll mutate themselves.
+2. State will always be serializable allowing you to store and rehydrate from browser storage mechanisms like local storage.
+3. State can be inspected via the Redux Devtools.
+
+This is one of the [core principles](docs#core-principles) of NgRx. The [Redux docs](https://redux.js.org/faq/organizingstate#can-i-put-functions-promises-or-other-non-serializable-items-in-my-store-state) also offers some more insight into this constraint.