You want to add a script node to your scene. Window > animation editors > expression editor. under the select filter menu change to "by script node name". type a name, in the top field, cxhange execte on to gui open/close, and in the script field add:
confirmDialog -m "Put your credit here";
hit create and it should pop up whenever someone opens the scene. Do note that under the open scene options, there is a setting to turn off script nodes. By default it is on, but if anyone who has turned it off opens the scene they will not see your pop up.
Hope that helps