Add example mounting ConfigMap as a single file into /etc dir

ref: https://github.com/kubernetes/kubernetes/pull/22575

docs/user-guide/configmap/README.md

@@ -101,3 +101,6 @@ This pod runs a `cat` command to print the value of one of the keys in the volum
 $ kubectl logs config-volume-test-pod
 value-1
 ```
+
+Alternatively you can use [`mount-file-pod.yaml`](mount-file-pod.yaml) file to mount
+only a file from ConfigMap, preserving original content of /etc directory.

docs/user-guide/configmap/mount-file-pod.yaml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: config-single-file-volume-pod
+spec:
+  containers:
+    - name: test-container
+      image: gcr.io/google_containers/busybox
+      command: [ "/bin/sh", "-c", "cat /etc/special-key" ]
+      volumeMounts:
+      - name: config-volume
+        mountPath: /etc/special-key
+        subPath: path/to/special-key
+  volumes:
+    - name: config-volume
+      configMap:
+        name: test-configmap
+        items:
+        - key: data-1
+          path: path/to/special-key
+  restartPolicy: Never