add ability to mount single config file

Change-Id: I328d73b6ee80410ad633fded1ef35d3ceeb77f2a
diff --git a/README.rst b/README.rst
index 0ee5fa5..e2be335 100644
--- a/README.rst
+++ b/README.rst
@@ -568,6 +568,19 @@
               key: policy.json
               path: policy.json
 
+To mount single configuration file instead of whole directory:
+
+.. code-block:: yaml
+
+  service:
+    memcached:
+      container:
+        memcached:
+          volumes:
+            - name: volume1
+              mountPath: /volume/config.conf
+              sub_path: config.conf
+
 Generating Jobs
 ===============