There are two ways. Registered Mindahome home owners can leave references in the form of reviews on a sitters profile page. To be able to do this the house sitter needs to send the home owner a review invitation.
All other references and documents from other sources can be saved as document files and uploaded to the "Files" page of the sitters Mindahome control panel where they can be added to the bottom of their profile page and also attached to messages they send. There are instructions on the "How to" page found in the Mindahome members control panel.