Add limitation that Cilium is incompatible with Istio

Author: Will Daly

articles/aks/azure-cni-powered-by-cilium.md

@@ -45,6 +45,7 @@ Azure CNI powered by Cilium currently has the following limitations:
 * Available only for Linux and not for Windows.
 * Cilium L7 policy enforcement is disabled.
 * Hubble is disabled.
+* Not yet configured for compatibility with Istio ([Istio issue #27619](https://github.com/istio/istio/issues/27619)).
 * Kubernetes services with `internalTrafficPolicy=Local` aren't supported ([Cilium issue #17796](https://github.com/cilium/cilium/issues/17796)).
 * Multiple Kubernetes services can't use the same host port with different protocols (for example, TCP or UDP) ([Cilium issue #14287](https://github.com/cilium/cilium/issues/14287)).
 * Network policies may be enforced on reply packets when a pod connects to itself via service cluster IP ([Cilium issue #19406](https://github.com/cilium/cilium/issues/19406)).