RANGER-5682: Add README for agents-common module

agents-common/README.md

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+<!---
+Licensed to the Apache Software Foundation (ASF) under one
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+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
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+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
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+-->
+
+# ranger-plugins-common (`agents-common`)
+
+Core policy engine and shared library for all Apache Ranger service plugins. Every Ranger authorization plugin (HDFS, Hive, Kafka, HBase, etc.) depends on this module to evaluate access requests against centrally managed policies.
+
+## How It Works
+
+Service plugins extend `RangerBasePlugin`, which handles:
+
+1. **Policy synchronization** - background download of policies from Ranger Admin via `PolicyRefresher`
+2. **Access evaluation** - request preprocessing (enrichment with tags, user-store, GDS metadata), zone-aware policy lookup via `RangerResourceTrie`, and per-policy evaluation
+3. **Audit generation** - producing audit events routed to configured destinations
+
+## Usage
+
+```java
+// Create and initialize plugin
+RangerHdfsPlugin plugin = new RangerHdfsPlugin(configFile);
+plugin.init();
+
+// Evaluate access
+Map<String, Object> resource = new HashMap<>();
+resource.put("path", "/data/finance");
+
+RangerAccessRequestImpl request = new RangerAccessRequestImpl();
+request.setResource(new RangerAccessResourceImpl(resource));
+request.setUser("alice");
+request.setUserGroups(new HashSet<>(Arrays.asList("finance-team")));
+request.setAccessType("read");
+
+RangerAccessResult result = plugin.isAccessAllowed(request);
+// result.getIsAllowed(), result.getPolicyId(), result.getMaskType()
+
+// Shutdown
+plugin.cleanup();
+```
+
+For a real integration example, see `hdfs-agent/src/main/java/org/apache/ranger/authorization/hadoop/RangerHdfsPlugin.java`.
+
+## Service Definitions
+
+JSON service definitions in `src/main/resources/service-defs/` declare the resource hierarchy, access types, policy conditions, and context enrichers for each supported service:
+
+HDFS, Hive, HBase, Kafka, Knox, Solr, Storm, YARN, Sqoop, Kylin, Elasticsearch, Atlas, KMS, Ozone, NiFi, NiFi-Registry, Presto, Trino, Schema-Registry, Kudu, WASB, ABFS, Tag, GDS, NestedStructure, Polaris.
+
+## Building
+
+```bash
+# Build this module and its dependencies (from the repository root)
+mvn clean install -pl agents-common -am -DskipTests
+
+# Rebuild only this module (after dependencies are already built)
+mvn clean install -pl agents-common -DskipTests
+
+# Run tests
+mvn test -pl agents-common -am
+```
+
+## Testing
+
+Tests are data-driven - JSON fixtures in `src/test/resources/policyengine/` define policies, access requests, and expected results. The engine evaluates each fixture and asserts the outcome matches.
+
+## Adding as a Dependency
+
+For plugin developers building a new Ranger authorization plugin:
+
+```xml
+<dependency>
+  <groupId>org.apache.ranger</groupId>
+  <artifactId>ranger-plugins-common</artifactId>
+  <version>${ranger.version}</version>
+</dependency>
+```