[DOCS] Intro for adding nodes (#37202)

docs/reference/setup.asciidoc

@@ -58,3 +58,5 @@ include::setup/bootstrap-checks.asciidoc[]
 include::setup/starting.asciidoc[]
 
 include::setup/stopping.asciidoc[]
+
+include::setup/add-nodes.asciidoc[]

docs/reference/setup/add-nodes.asciidoc

@@ -0,0 +1,37 @@
+[[add-elasticsearch-nodes]]
+== Adding nodes to your cluster
+
+When you start an instance of {es}, you are starting a _node_. An {es} _cluster_
+is a group of nodes that have the same `cluster.name` attribute. As nodes join
+or leave a cluster, the cluster automatically reorganizes itself to evenly
+distribute the data across the available nodes.
+
+If you are running a single instance of {es}, you have a cluster of one node.
+All primary shards reside on the single node. No replica shards can be
+allocated, therefore the cluster state remains yellow. The cluster is fully
+functional but is at risk of data loss in the event of a failure.
+
+image::setup/images/elas_0202.png["A cluster with one node and three primary shards"]
+
+You add nodes to a cluster to increase its capacity and reliability. By default,
+a node is both a data node and eligible to be elected as the master node that
+controls the cluster. You can also configure a new node for a specific purpose,
+such as handling ingest requests. For more information, see
+<<modules-node,Nodes>>.
+
+When you add more nodes to a cluster, it automatically allocates replica shards.
+When all primary and replica shards are active, the cluster state changes to
+green.
+
+image::setup/images/elas_0204.png["A cluster with three nodes"]
+
+To add a node to a cluster:
+
+. Set up a new {es} instance.
+. Specify the name of the cluster in its `cluster.name` attribute. For example,
+to add a node to the `logging-prod` cluster, set `cluster.name: "logging-prod"`
+in `elasticsearch.yml`.
+. Start {es}. The node automatically discovers and joins the specified cluster.
+
+For more information about discovery and shard allocation, see
+<<modules-discovery>> and <<modules-cluster>>.