add prerequisit to have permissions to create service principals in t…

…he subscription (Azure#2281)

acs-engine hangs with "WARN[0008] apimodel: ServicePrincipalProfile was empty, assigning role to application..."
if user does not have enough permissions to create and assign service principals ans azure applications

docs/kubernetes/deploy.md

@@ -19,6 +19,7 @@ After the cluster is deployed the upgrade and [scale](scale.md) commands can be
 ## Gather Information
 
 * The subscription in which you would like to provision the cluster. This is a uuid which can be found with `az account list -o table`.
+* Proper access rights within the subscription. Especially the right to create and assign service principals to applications ( see  ACS Engine the Long Way, Step #2)
 * A `dnsPrefix` which forms part of the the hostname for your cluster (e.g. staging, prodwest, blueberry). The DNS prefix must be unique so pick a random name.
 * A location to provision the cluster e.g. `westus2`.