Update Resize-AzureRmVirtualNetworkGateway.md

Adding a note that was made by Charley Peng in the docs repo copy of this older MD to the source repo version of this MD.

Author: TaraMeyer

src/ResourceManager/Network/Commands.Network/help/Resize-AzureRmVirtualNetworkGateway.md

@@ -96,6 +96,7 @@ This cmdlet accepts pipelined instances of the **Microsoft.Azure.Commands.Networ
 This cmdlet modifies existing instances of the **Microsoft.Azure.Commands.Network.Models.PSVirtualNetworkGateway** object.
 
 ## NOTES
+You cannot resize from Basic/Standard/HighPerformance SKUs to the new VpnGw1/VpnGw2/VpnGw3 SKUs. See https://docs.microsoft.com/en-us/azure/vpn-gateway/vpn-gateway-about-vpngateways for instructions.
 
 ## RELATED LINKS