Update http-proxy.md

Adding a limitation that the http proxy feature can't be enabled on existing cluster and it can be enabled on cluster creation time.

Author: mutazn

articles/aks/http-proxy.md

@@ -24,6 +24,7 @@ The following scenarios are **not** supported:
 * Different proxy configurations per node pool
 * User/Password authentication
 * Custom certificate authorities (CAs) for API server communication
+* Configuring existing AKS clusters with an HTTP proxy feature is not supported; the HTTP proxy feature must be enabled at cluster creation time.
 * Windows-based clusters
 * Node pools using Virtual Machine Availability Sets (VMAS)
 * Using * as wildcard attached to a domain suffix for noProxy