Update comments on sample.env file (Azure#20440)

This commit adds more comments on the sample.env file to specify that an App Configuration resource is needed and other env variables too.

Author: JonathanCrd

sdk/template/template/sample.env

@@ -1,7 +1,14 @@
-# Used in most samples. Retrieve these values from an instance in the Azure
-# Portal. The APPCONFIG_TEST_SETTING_KEY value indicates which key to use
-# when retrieving an example setting.
+# Used in most samples. 
+# The template project is based on the App Configuration SDK.
+
+# Retrieve the following values from an App Configuration instance in the Azure Portal.
 
 APPCONFIG_ENDPOINT="https://<resource name>.azconfig.io",
 APPCONFIG_TEST_SETTING_KEY="<key>"
+# The APPCONFIG_TEST_SETTING_KEY value indicates which key to use when retrieving an example setting.
+
+# Retrieve the following values from your app registration in the Azure Portal.
 
+AZURE_TENANT_ID=""
+AZURE_CLIENT_ID=""
+AZURE_CLIENT_SECRET=""