add matchLabels description (kubernetes#6020)

docs/concepts/workloads/controllers/deployment.md

@@ -48,6 +48,10 @@ In this example:
   [Docker Hub](https://hub.docker.com/) image at version 1.7.9.
 * The Deployment opens port 80 for use by the Pods.
 
+Note: `matchLabels` is a map of {key,value} pairs. A single {key,value} in the matchLabels map 
+is equivalent to an element of matchExpressions, whose key field is "key", the operator is "In",
+and the values array contains only "value". The requirements are ANDed.
+
 The `template` field contains the following instructions:
 
 * The Pods are labeled `app: nginx`