Updated user_guide for async client (#338)

Signed-off-by: saimedhi <saimedhi@amazon.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Removed
 ### Fixed
 - Fixed import cycle when importing async helpers ([#311](https://github.com/opensearch-project/opensearch-py/pull/311))
+- Fixed userguide for async client ([#311](https://github.com/opensearch-project/opensearch-py/pull/311))
 ### Security
 - Fixed CVE-2022-23491 reported in opensearch-dsl-py ([#295](https://github.com/opensearch-project/opensearch-py/pull/295))
 - Update ci workflows ([#318](https://github.com/opensearch-project/opensearch-py/pull/318))

USER_GUIDE.md

@@ -478,6 +478,10 @@ print(response)
 
 Make sure to use `AsyncOpenSearch` with the `AsyncHttpConnection` connection class with the async `AWSV4SignerAsyncAuth` signer.
 
+```bash
+pip install opensearch-py[async]
+```
+
 ```python
 from opensearchpy import AsyncOpenSearch, AsyncHttpConnection, AWSV4SignerAsyncAuth
 import boto3