Guide app dev to setup CLIENT_SECRET env var

Author: rayluo

README.md

@@ -128,8 +128,9 @@ In the steps below, "ClientID" is the same as "Application ID" or "AppId".
 
 1. Open the `app_config.py` file
 1. Find the app key `Enter_the_Tenant_Name_Here` and replace the existing value with your Azure AD tenant name.
-1. You could find the app key `Enter_the_Client_Secret_Here` and replace the existing value with the key you saved during the creation of the `python-webapp` app, in the Azure portal.
-   But we recommend you to store the secret in environment variable, rather than in file.
+1. You saved your application secret during the creation of the `python-webapp` app in the Azure portal.
+   Now you can set the secret in environment variable `CLIENT_SECRET`,
+   which will then be picked up by this sample app.
 1. Find the app key `Enter_the_Application_Id_here` and replace the existing value with the application ID (clientId) of the `python-webapp` application copied from the Azure portal.