Add Note about connection dependencies

Added a note advising users to check for possible connection dependencies from other resources that may prevent the management of a subnet.

Author: jeremywilley1024

articles/virtual-network/virtual-network-manage-subnet.md

@@ -74,6 +74,15 @@ To do tasks on subnets, your account must be assigned to the [Network contributo
 | Microsoft.Network/virtualNetworks/subnets/joinViaServiceEndpoint/action | Enable a service endpoint for a subnet.     |
 | Microsoft.Network/virtualNetworks/subnets/virtualMachines/read          | Get the virtual machines in a subnet.       |
 
+[!NOTE]
+If attemtps to add, change, or delete a subnet in your virtual network fail, ensure there are no connections to other resources that may block the action:
+
+1. Gateway connections
+2. Gateways
+3. IPs
+4. Virtual network peerings
+5. App Service Environment (ASE)
+
 ## Add a subnet
 
 # [Portal](#tab/azure-portal)