[DOCS] Adding image to 'basic write model' section.

docs/reference/docs/data-replication.asciidoc

@@ -28,6 +28,8 @@ Every indexing operation in Elasticsearch is first resolved to a replication gro
 typically based on the document ID. Once the replication group has been determined, the operation is forwarded
 internally to the current _primary shard_ of the group. This stage of indexing is referred to as the _coordinating stage_.
 
+image::images/data_processing_flow.png[An example of a basic write model.]
+
 The next stage of indexing is the _primary stage_, performed on the primary shard. The primary shard is responsible
 for validating the operation and forwarding it to the other replicas. Since replicas can be offline, the primary
 is not required to replicate to all replicas. Instead, Elasticsearch maintains a list of shard copies that should