Add README to async-query-core (#2766)

Signed-off-by: Tomoyuki Morita <[email protected]>

async-query-core/README.md

@@ -0,0 +1,32 @@
+# async-query-core library
+
+This directory contains async-query-core library, which implements the core logic of async-query and provide extension points to allow plugin different implementation of data storage, etc.
+`async-query` module provides implementations for OpenSearch index based implementation.
+
+## Type of queries
+There are following types of queries, and the type is automatically identified by analysing the query. 
+- BatchQuery: Execute single query in Spark
+- InteractiveQuery: Establish session and execute queries in single Spark session
+- IndexDMLQuery: Handles DROP/ALTER/VACUUM operation for Flint indices
+- RefreshQuery: One time query request to refresh(update) Flint index
+- StreamingQuery: Continuously update flint index in single Spark session
+
+## Extension points
+Following is the list of extension points where the consumer of the library needs to provide their own implementation.
+
+- Data store interface
+  - [AsyncQueryJobMetadataStorageService](src/main/java/org/opensearch/sql/spark/asyncquery/AsyncQueryJobMetadataStorageService.java)
+  - [SessionStorageService](java/org/opensearch/sql/spark/execution/statestore/SessionStorageService.java)
+  - [StatementStorageService](src/main/java/org/opensearch/sql/spark/execution/statestore/StatementStorageService.java)
+  - [FlintIndexMetadataService](src/main/java/org/opensearch/sql/spark/flint/FlintIndexMetadataService.java)
+  - [FlintIndexStateModelService](src/main/java/org/opensearch/sql/spark/flint/FlintIndexStateModelService.java)
+  - [IndexDMLResultStorageService](src/main/java/org/opensearch/sql/spark/flint/IndexDMLResultStorageService.java)
+- Other
+  - [LeaseManager](src/main/java/org/opensearch/sql/spark/leasemanager/LeaseManager.java)
+  - [JobExecutionResponseReader](src/main/java/org/opensearch/sql/spark/response/JobExecutionResponseReader.java)
+  - [QueryIdProvider](src/main/java/org/opensearch/sql/spark/dispatcher/QueryIdProvider.java)
+  - [SessionIdProvider](src/main/java/org/opensearch/sql/spark/execution/session/SessionIdProvider.java)
+  - [SessionConfigSupplier](src/main/java/org/opensearch/sql/spark/execution/session/SessionConfigSupplier.java)
+  - [SparkExecutionEngineConfigSupplier](src/main/java/org/opensearch/sql/spark/config/SparkExecutionEngineConfigSupplier.java)
+  - [SparkSubmitParameterModifier](src/main/java/org/opensearch/sql/spark/config/SparkSubmitParameterModifier.java)
+  - [EMRServerlessClientFactory](src/main/java/org/opensearch/sql/spark/client/EMRServerlessClientFactory.java)