Update node.asciidoc

Master nodes must have access to a persistent data directory

Author: clintongormley

docs/reference/modules/node.asciidoc

@@ -82,6 +82,9 @@ cluster health to have a stable master node.
 Any master-eligible node (all nodes by default) may be elected to become the
 master node by the <<modules-discovery-zen,master election process>>.
 
+IMPORTANT: Master nodes must still access to the `data/` directory (just like 
+`data` nodes) as this is where the cluster state is persisted between node restarts.
+
 Indexing and searching your data is CPU-, memory-, and I/O-intensive work
 which can put pressure on a node's resources. To ensure that your master
 node is stable and not under pressure, it is a good idea in a bigger