Add clarification to Alertmanager support policy

Current wording conveys the impression that creating additional receivers is not supported which is incorrect, as confirmed by the Red Hat SRE team.

Author: andykrohg

articles/openshift/support-policies-v4.md

@@ -22,7 +22,8 @@ Certain configurations for Azure Red Hat OpenShift 4 clusters can affect your cl
 * The cluster must have a minimum of three worker nodes and three manager nodes.
 * Don't scale the cluster workers to zero, or attempt a cluster shutdown. Deallocating or powering down any virtual machine in the cluster resource group is not supported.
 * Don't have taints that prevent OpenShift components to be scheduled.
-* Don't remove or modify the cluster Prometheus and Alertmanager services.
+* Don't remove or modify the cluster Prometheus service.
+* Don't remove or modify the cluster Alertmanager service or Default receiver. It *is* supported to create additional receivers to notify external systems.
 * Don't remove Service Alertmanager rules.
 * Security groups can't be modified. Any attempt to modify security groups will be reverted.
 * Don't remove or modify Azure Red Hat OpenShift service logging (mdsd pods).
@@ -126,4 +127,4 @@ Azure Red Hat OpenShift 4 supports node instances on the following virtual machi
 |Series|Size|vCPU|Memory: GiB|
 |-|-|-|-|
 |G|Standard_G5|32|448|
-|G|Standard_GS5|32|448|
+|G|Standard_GS5|32|448|