Add USER_GUIDE.md

USER_GUIDE.md

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+# User Guide
+
+- [User Guide](#user-guide)
+  - [Sample data](#sample-data)
+  - [Create an index](#create-an-index)
+  - [Index data](#index-data)
+  - [Search for the document](#search-for-the-document)
+  - [Search documents using a match query](#search-documents-using-a-match-query)
+  - [Aggregations](#aggregations)
+  - [Delete the document](#delete-the-document)
+  - [Delete the index](#delete-the-index)
+  - [Aggregations](#aggregations)
+
+## Sample data
+
+### IndexData class
+
+```java
+static class IndexData {
+  private String firstName;
+  private String lastName;
+
+  public IndexData(String firstName, String lastName) {
+    this.firstName = firstName;
+    this.lastName = lastName;
+  }
+
+  public String getFirstName() {
+    return firstName;
+  }
+
+  public void setFirstName(String firstName) {
+    this.firstName = firstName;
+  }
+
+  public String getLastName() {
+    return lastName;
+  }
+
+  public void setLastName(String lastName) {
+    this.lastName = lastName;
+  }
+
+  @Override
+  public String toString() {
+    return String.format("IndexData{first name='%s', last name='%s'}", firstName, lastName);
+  }
+}
+```
+
+## Create an index
+
+```java
+String index = "sample-index";
+CreateIndexRequest createIndexRequest = new CreateIndexRequest.Builder().index(index).build();
+client.indices().create(createIndexRequest);
+```
+
+## Index data
+
+```java
+IndexData indexData = new IndexData("John", "Doe");
+IndexRequest<IndexData> indexRequest = new IndexRequest.Builder<IndexData>().index(index).id("1").document(indexData).build();
+client.index(indexRequest);
+
+indexData = new IndexData("John", "Joe");
+indexRequest = new IndexRequest.Builder<IndexData>().index(index).id("2").document(indexData).build();
+client.index(indexRequest);
+```
+
+## Search for the documents
+
+```java
+SearchResponse<IndexData> searchResponse = client.search(s -> s.index(index), IndexData.class);
+for (int i = 0; i < searchResponse.hits().hits().size(); i++) {
+  System.out.println(searchResponse.hits().hits().get(i).source());
+}
+```
+
+## Search documents using a match query
+
+```java
+SearchRequest searchRequest = new SearchRequest.Builder().query(q -> q.match(m -> m.field("firstName")
+                                                                                   .query(FieldValue.of("John"))))
+                                                         .build();
+
+SearchResponse<IndexData> searchResponse = client.search(searchRequest, IndexData.class);
+for (int i = 0; i < searchResponse.hits().hits().size(); i++) {
+  System.out.println(searchResponse.hits().hits().get(i).source());
+}
+```
+
+## Aggregations
+
+```java
+SearchRequest searchRequest = new SearchRequest.Builder().query(q -> q.match(m -> m.field("firstName")
+                                                                                   .query(FieldValue.of("John"))))
+                                                         .aggregations("firstNames", new Aggregation.Builder().terms(t -> t.field("firstName.keyword"))
+                                                                                                              .build())
+                                                         .build();
+
+SearchResponse<IndexData> searchResponse = client.search(searchRequest, IndexData.class);
+for (Map.Entry<String, Aggregate> entry : searchResponse.aggregations().entrySet()) {
+  System.out.println("Agg - " + entry.getKey());
+  entry.getValue().sterms().buckets().array().forEach(b -> System.out.printf("%s : %d%n", b.key(), b.docCount()));
+}
+```
+
+## Delete the document
+
+The following sample code deletes a document whose ID is 1.
+
+```java
+client.delete(d -> d.index(index).id("1"));
+```
+
+## Delete the index
+
+```java
+DeleteIndexRequest deleteIndexRequest = new DeleteRequest.Builder().index(index).build();
+DeleteIndexResponse deleteIndexResponse = client.indices().delete(deleteIndexRequest);
+```