added explanation of usage

au/update_vars_default.ps1

@@ -1,4 +1,5 @@
 # AU Packages Template: https://github.com/majkinetor/au-packages-template
+# Copy this file to update_vars.ps1 and set the variables there. Do not include it in the repository.
 
 $Env:mail_user        = ''
 $Env:mail_pass        = ''