Update associate-public-ip-address-vm.md

The following is one of the questions asked in Azure Administrator Certification exam and should be clearly stated in the doc (I haven't seen it anywhere in the doc)

"To share a VM with an external user, you must add a public IP address to the VM."

Meanwhile, I added about Azure Bastion so readers have a more holistic view about their options.

Author: azarboon

articles/virtual-network/ip-services/associate-public-ip-address-vm.md

@@ -218,6 +218,8 @@ Install [Azure PowerShell](/powershell/azure/install-azure-powershell) on your m
 1. Open the necessary ports in your security groups by adjusting the security rules in the network security groups. For information, see [Allow network traffic to the VM](#allow-network-traffic-to-the-vm).
 
 ---
+> [!NOTE]  
+> To share a VM with an external user, you must add a public IP address to the VM. Alternatively, external users can connect to VM's private IP address through Azure Bastion.
 
 ## Allow network traffic to the VM