docs: Document reduced API call

Author: sbernauer

README.md

@@ -31,10 +31,47 @@ The Stackable HDFS already takes care of this, you don't need to do anything in
 
 ### API
 
-For every action a request similar to the following is sent to OPA:
+By default for every HDFS action a request similar to the following is sent to OPA:
+
+
+```json
+{
+  "input": {
+    "fsOwner": "nn",
+    "supergroup": "supergroup",
+    "callerUgi": {
+      "realUser": null,
+      "userName": "alice/[email protected]",
+      "shortUserName": "alice",
+      "primaryGroup": "developers",
+      "groups": [
+        "developers"
+      ],
+      "authenticationMethod": "KERBEROS",
+      "realAuthenticationMethod": "KERBEROS"
+    },
+    "snapshotId": 2147483646,
+    "path": "/developers-ro/hosts._COPYING_",
+    "ancestorIndex": 1,
+    "doCheckOwner": false,
+    "ignoreEmptyDir": false,
+    "operationName": "getfileinfo",
+    "callerContext": {
+      "context": "CLI",
+      "signature": null
+    }
+  }
+}
+```
+
+The contained details should be sufficient for most use-cases.
+However, if you need access to all the provided information from the `INodeAttributeProvider.AccessControlEnforcer` interface, you can instruct hdfs-utils to send all fields by setting `hadoop.security.authorization.opa.extended-requests` to `true`.
+However, please note that this results in very big JSON objects being send from HDFS to OPA, so please keep an eye on performance degradations.
+
+The following example provides an extend request sending all available fields:
 
 <details>
-<summary>Example request</summary>
+<summary>Example extended request</summary>
 
 ```json
 {