Clarify where resolution works.

cluster/addons/dns/README.md

@@ -13,6 +13,11 @@ crashes or scheduling changes).  This maps well to DNS, which has a long
 history of clients that, on purpose or on accident, do not respect DNS TTLs
 (see previous remark about Pod IPs changing).
 
+## Where does resolution work?
+Kubernetes Service DNS names can be resolved using standard methods (e.g. [`gethostbyname`](
+http://linux.die.net/man/3/gethostbyname)) inside any pod, except pods which
+have the `hostNet` field set to `true`.
+
 ## Supported DNS schema
 The following sections detail the supported record types and layout that is
 supported.  Any other layout or names or queries that happen to work are