Update app-only-auth-powershell-v2.md

Adding note that this feature is currently in Public Preview and the required installation string.

Author: navgupta1208

exchange/docs-conceptual/app-only-auth-powershell-v2.md

@@ -59,6 +59,13 @@ The following examples show how to use the Exchange Online PowerShell V2 module
 
 The EXO V2 module uses the Active Directory Authentication Library to fetch an app-only token using the application Id, tenant Id & certificate thumbprint. The application object provisioned inside Azure AD has a Directory Role assigned to it, which is returned in the access token. Exchange Online configures the session RBAC using the directory role information that's available in the token.
 
+## How to start using this feature ?
+Please note that this feature is currently in Public Preview and available in the Preview release of EXO PowerShell V2 Module. You need to use below strings to install the Preview version 2.0.3 which contains this feature - 
+
+     ```powershell
+     Install-Module -Name ExchangeOnlineManagement -RequiredVersion 2.0.3-Preview -AllowPrerelease
+     ```
+
 ## Setup app-only authentication
 
 An initial onboarding is required for authentication using application objects. Application and service principal are used interchangeably, but an application is like a class object while a service principal is like an instance of the class. You can learn more about this at [Application and service principal objects in Azure Active Directory](https://docs.microsoft.com/azure/active-directory/develop/app-objects-and-service-principals).