Update features.md
MSI feature is not ready in k8s 1.8 since it requires azure-sdk-for-go version upgrade, this issue is fixed in v1.9, you could find detailed info here:
Enabling Managed Identity configures acs-engine to include and use MSI identities for all interactions with the Azure Resource Manager (ARM) API.
-Instead of using a static servic principal written to `/etc/kubernetes/azure.json`, Kubernetes will use a dynamic, time-limited token fetched from the MSI extension running on master and agent nodes. This support is currently alpha and requires Kubernetes v1.7.2 or newer.
+Instead of using a static servic principal written to `/etc/kubernetes/azure.json`, Kubernetes will use a dynamic, time-limited token fetched from the MSI extension running on master and agent nodes. This support is currently alpha and requires Kubernetes v1.9.1 or newer.
Enable Managed Identity by adding `useManagedIdentity` in `kubernetesConfig`.
