Add a feature to an open-source project and 'build' it for my MacBook

I am a user of Manuscripts.io, and I wish to use it to write a paper. However, it does not have a footnote feature which is critical for me. The project is open source (https://gitlab.com/mpapp-public). I am looking for someone to add the footnote feature (which apparently used to exist), and then make it so that I can use the software offline on my MacBook.