You can provide a template for Copyright document for Authors to refer while you are creating a Submix page. 

Once the Submix page has been created, you can still update/add the copywright document by following these steps:
Login >> Submix >> Click "manage" on the Submix page >> Edit >> Scroll down >> Upload Copyright document >> Save                                

Once you add this, this document will be available on the Submix page

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