Include supported version skew

A quick fix until the version skew is properly documented for master/node and master/kubectl, per kubernetes#7103.

docs/tasks/tools/install-kubectl.md

@@ -10,7 +10,7 @@ Use the Kubernetes command-line tool, [kubectl](/docs/user-guide/kubectl/), to d
 {% endcapture %}
 
 {% capture prerequisites %}
-Use a version of kubectl that is the same version as your server or later. Using an older kubectl with a newer server might produce validation errors.
+Use a version of kubectl that is the same version as your server or later. Using an older kubectl with a newer server might produce validation errors.  Only 1 minor version skew is supported between kubectl and the server (e.g. v1.9 client is supported on v1.8 and v1.9 servers, but not on v1.7 server). 
 {% endcapture %}
 
 ## Install kubectl