The license is MIT, you don't need to add anything else.
> The >300 open issues and what seems like tons of ignored pull-requests
Old version of PhotoSwipe was not developed me and I did not manage repository. Previous owners just passed development to me a few months ago https://twitter.com/PhotoSwipe/status/444134042787930113 and I started working on a new version.
I can assure you that all new issues and pull-requests will be managed appropriately.