Hello @Jan_Hartwig,
If you want to add some text in the uploader view, you can use below filter:
add_filter('rtmedia_uploader_before_start_upload_button','add_custom_message'); function add_custom_message(){ return "Custom Message of your choice"; }
If you want to select any other place to display this message, you can find other filters in file here : https://github.com/rtCamp/rtMedia/blob/master/app/main/controllers/upload/RTMediaUploadView.php#L41
Thank you.