Add doc about supporting donotschedule only

keps/sig-scheduling/3022-min-domains-in-pod-topology-spread/README.md

@@ -111,7 +111,7 @@ type TopologySpreadConstraint struct {
   // Pod Topology Spread treats "global minimum" as 0.
   // As a result, when the number of domains is less than `minDomains`,
   // scheduler doesn't schedule a matching Pod to Nodes on the domains that have the same or more number of matching Pods as `maxSkew`.
-  // Default value is 0.
+  // Default value is 0. When value is different than 0, WhenUnsatisfiable must be DoNotSchedule.
   // +optional
   MinDomains *int32
 }