Update doc

docs/tutorial/PreviewFeatures.md

@@ -23,3 +23,28 @@ Here is an example to disable this feature by setting the property "indexer-deci
 
 If the property "indexer-decimation-enabled" is not created or the property value is set to "true" (String type) in the 
 given data partition, the geo-shape decimation will be enabled.
+
+## Index extension
+
+OSDU Standard index extensions are defined by OSDU Data Definition work-streams with the intent to provide
+user/application friendly, derived properties. The standard set, together with the OSDU schemas, form the
+interoperability foundation. They can contribute to deliver domain specific APIs according to the Domain Driven Design
+principles.
+
+The configurations are encoded in OSDU reference-data records, one per each major schema version. The type name
+is IndexPropertyPathConfiguration. With this, the extension properties can be defined as if they were provided by a schema.
+
+In order to reduce the risk when extended evaluation of the solution is still on going, a feature flag that is managed by
+the Partition Service is applied to the solution. Here is an example to enable this feature by setting the property 
+"index-augmenter-enabled" in a given data partition:
+```
+{
+   "index-augmenter-enabled": {
+        "sensitive": false,
+        "value": "true"
+    }
+}
+```
+
+If the property "index-augmenter-enabled" is not created or the property value is set to "false" (String type) in the
+given data partition, the configurations defined as type IndexPropertyPathConfiguration will be ignored and index extension will be disabled.