“Scribus.app” can’t be opened because it is from an unidentified developer.

I downloaded Scribus, an open source free page layout design application from https://www.scribus.net/ [https://www.scribus.net/ link text] I can't open this application because I get a message that says: "'Scribus.app' can’t be opened because it is from an unidentified developer. Your security preferences allow installation of only apps from the Mac App Store and identified developers.'' I searched the Mac App Store and it does not have Scribus. I can't afford to subscribe to Adobe Creative Cloud and I need to be able to create page layout designs, which Scribus can do. How can I verify that https://www.scribus.net/ is a secure application site? How can I open the application if it is a secure link? Thanks, Shelley