Document how to use Java 8 Time (JSR 310) objects

Signed-off-by: Daniel Widdis <widdis@gmail.com>

CHANGELOG.md

@@ -16,7 +16,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Add timeout and throttle to the jenkins workflows ([#231](https://github.com/opensearch-project/opensearch-java/pull/231)) 
 - Updating maintainers, admins and documentation ([#248](https://github.com/opensearch-project/opensearch-java/pull/248))
 - Migrate client transports to Apache HttpClient / Core 5.x ([#246](https://github.com/opensearch-project/opensearch-java/pull/246))
-- Add support for JSR-310 Date and Time API classes ([#251](https://github.com/opensearch-project/opensearch-java/pull/251))
+- Document how to use Java 8 Time (JSR 310) objects ([#251](https://github.com/opensearch-project/opensearch-java/pull/251))
 - Fixing version and build ([#254](https://github.com/opensearch-project/opensearch-java/pull/254))
 
 ### Deprecated

USER_GUIDE.md

@@ -2,9 +2,10 @@
 
 - [User Guide](#user-guide)
   - [Sample data](#sample-data)
+  - [Create a client](#create-a-client)
   - [Create an index](#create-an-index)
   - [Index data](#index-data)
-  - [Search for the document](#search-for-the-document)
+  - [Search for the document](#search-for-the-documents)
   - [Search documents using a match query](#search-documents-using-a-match-query)
   - [Aggregations](#aggregations)
   - [Delete the document](#delete-the-document)
@@ -48,6 +49,25 @@ static class IndexData {
 }
 ```
 
+## Create a client
+
+```java
+Transport transport = new RestClientTransport(restClient, new JacksonJsonpMapper()); 
+OpenSearchClient client = new OpenSearchClient(transport);
+```
+
+The `JacksonJsonpMapper` class (2.x versions) only supports Java 7 objects by default.
+[Java 8 modules](https://github.com/FasterXML/jackson-modules-java8) to support JDK8 classes
+such as the Date and Time API (JSR-310), `Optional`, and more can be used by including
+[the additional datatype dependency](https://github.com/FasterXML/jackson-modules-java8#usage)
+and adding the module.  For example, to include JSR-310 classes:
+
+```java
+Transport transport = new RestClientTransport(restClient,
+    new JacksonJsonpMapper(new ObjectMapper().registerModule(new JavaTimeModule()))); 
+OpenSearchClient client = new OpenSearchClient(transport);
+```
+
 ## Create an index
 
 ```java