Include a small introduction text to email on loading files
    
    Basically, this introduce the rest of the email which will contain
    the log about loading the JSON files pushed to the git repositories
    to the DB, thus allowing to update tickets/PRs from the git repos.
    
    Fixes https://pagure.io/pagure/issue/4003
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>