added extensions limit

Added 'VirtualMachines/extensions' under Microsoft.Compute as supporting an unlimited amount. Please validate that this is true or, if not, let me know the correct amount and I'll update it

Author: kegregoi

articles/azure-resource-manager/management/resources-without-resource-group-limit.md

@@ -43,6 +43,7 @@ Some resources have a limit on the number instances per region. This limit is di
 * snapshots
 * virtualMachineScaleSets - By default, limited to 800 instances. That limit can be increased by contacting support.
 * virtualMachines
+* virtualMachines/extensions - supports an unlimited number of VM extension instances
 
 ## Microsoft.ContainerInstance