Update GuidelinesGraph.md

Author: tylercleveland2

graph/GuidelinesGraph.md

@@ -14,6 +14,7 @@ Table of contents
     - [Query support](#query-support)
     - [Behavior modeling](#behavior-modeling)
     - [Error handling](#error-handling)
+    - [Limitations on core types](#limitations-on-core-types)
   - [External standards](#external-standards)
   - [API contract and nonbackward compatible changes](#api-contract-and-nonbackward-compatible-changes)
     - [Versioning and deprecation](#versioning-and-deprecation)
@@ -332,6 +333,15 @@ For a complete mapping of error codes to HTTP statuses, see
 
 <a name="api-contract-and-non-backward-compatible-changes"></a>
 
+### Limitations on core types
+
+The types `user`, `group`, and `device` should not have any new structural property(s) added, without compelling justification.
+Instead, model the concept represented in those property(s) as a new entity, and do one of the following:
+1. Add navigation to the new entity from `user`, `group`, or `device`.
+2. Contain the new entity elsewhere, and add a navigation from the new entity to `user`, `group` or `device`.
+
+More details and examples are available in [Core types](./articles/coreTypes.md).
+
 ## External standards
 
 For ease of client use and interoperatibility, some APIs might implement a standard that is defined external to Microsoft Graph and OData.